Before results appear in the UrlbarView, they are fetched from providers.

Each UrlbarProvider implements its own internal ranking and returns sorted results.

Externally all the results are ranked by the UrlbarMuxer according to an hardcoded list of groups and sub-grups.


Preferences can influence the groups order, for example by putting Firefox Suggest before Search Suggestions.

The Places provider, responsible to return history and bookmark results, uses an internal ranking algorithm called Frecency.

Frecency implementation

Frecency is a term derived from frequency and recency, its scope is to provide a ranking algorithm that gives importance both to how often a page is accessed and when it was last visited. Additionally, it accounts for the type of each visit through a bonus system.

To account for recency, a bucketing system is implemented. If a page has been visited later than the bucket cutoff, it gets the weight associated with that bucket:

  • Up to 4 days old - weight 100 - places.frecency.firstBucketCutoff/Weight

  • Up to 14 days old - weight 70 - places.frecency.secondBucketCutoff/Weight

  • Up to 31 days old - weight 50 - places.frecency.thirdBucketCutoff/Weight

  • Up to 90 days old - weight 30 - places.frecency.fourthBucketCutoff/Weight

  • Anything else - weight 10 - places.frecency.defaultBucketWeight

To account for frequency, the total number of visits to a page is used to calculate the final score.

The type of each visit is taken into account using specific bonuses:

Default bonus

Any unknown type gets a default bonus. This is expected to be unused. Pref places.frecency.defaultVisitBonus current value: 0.


Used for embedded/framed visits not due to user actions. These visits today are stored in memory and never participate to frecency calculation. Thus this is currently unused. Pref places.frecency.embedVisitBonus current value: 0.

Framed Link

Used for cross-frame visits due to user action. Pref places.frecency.framedLinkVisitBonus current value: 0.


Used for download visits. It’s important to support link coloring for these visits, but they are not necessarily useful address bar results (the Downloads view can do a better job with these), so their frecency can be low. Pref places.frecency.downloadVisitBonus current value: 0.


Used for reload visits (refresh same page). Low because it should not be possible to influence frecency by multiple reloads. Pref places.frecency.reloadVisitBonus current value: 0.

Redirect Source

Used when the page redirects to another one. It’s a low value because we give more importance to the final destination, that is what the user actually visits, especially for permanent redirects. Pref places.frecency.redirectSourceVisitBonus current value: 25.

Temporary Redirect

Used for visits resulting from a temporary redirect (HTTP 307). Pref places.frecency.tempRedirectVisitBonus current value: 40.

Permanent Redirect

Used for visits resulting from a permanent redirect (HTTP 301). This is the new supposed destination for a url, thus the bonus is higher than temporary. In this case it may be advisable to just pick the bonus for the source visit. Pref places.frecency.permRedirectVisitBonus current value: 50.


Used for visits generated from bookmark views. Pref places.frecency.bookmarkVisitBonus current value: 75.


Used for normal visits, for example when clicking on a link. Pref places.frecency.linkVisitBonus current value: 100.


Intended to be used for pages typed by the user, in reality it is used when the user picks a url from the UI (history views or the Address Bar). Pref places.frecency.typedVisitBonus current value: 2000.

The above bonuses are applied to visits, in addition to that there are also a few bonuses applied in case a page is not visited at all, both of these bonuses can be applied at the same time:

Unvisited bookmarked page

Used for pages that are bookmarked but unvisited. Pref places.frecency.unvisitedBookmarkBonus current value: 140.

Unvisited typed page

Used for pages that were typed and now are bookmarked (otherwise they would be orphans). Pref places.frecency.unvisitedTypedBonus current value: 200.

Two special frecency values are also defined:

  • -1 represents a just inserted entry in the database, whose score has not been calculated yet.

  • 0 represents an entry for which a new value should not be calculated, because it has a poor user value (e.g. place: queries) among search results.

Finally, because calculating a score from all of the visits every time a new visit is added would be expensive, only a sample of the last 10 (pref places.frecency.numVisits) visits is used.

How frecency for a page is calculated

flowchart TD start[URL] a0{Has visits?} a1[Get last 10 visit] a2[bonus = unvisited_bonus + bookmarked + typed] a3{bonus > 0?} end0[Frecency = 0] end1["frecency = age_bucket_weight * (bonus / 100)"] a4[Sum points of all sampled visits] a5{points > 0?} end2[frecency = -1] end3["Frecency = visit_count * (points / sample_size)"] subgraph sub [Per each visit] sub0[bonus = visit_type_bonus] sub1{bookmarked?} sub2[add bookmark bonus] sub3["score = age_bucket_weight * (bonus / 100)"] sub0 --> sub1 sub1 -- yes --> sub2 sub1 -- no --> sub3 sub2 --> sub3; end start --> a0 a0 -- no --> a2 a2 --> a3 a3 -- no --> end0 a3 -- yes --> end1 a0 -- yes --> a1 a1 --> sub sub --> a4 a4 --> a5 a5 -- no --> end2 a5 -- yes --> end3

Frecency calculation flow

  1. If the page is visited, get a sample of NUM_VISITS most recent visits.

  2. For each visit get a transition bonus, depending on the visit type.

  3. If the page is bookmarked, add to the bonus an additional bookmark bonus.

  4. If the bonus is positive, get a bucket weight depending on the visit date.

  5. Calculate points for the visit as age_bucket_weight * (bonus / 100).

  6. Sum points for all the sampled visits.

  7. If the points sum is zero, return a -1 frecency, it will still appear in the UI. Otherwise, frecency is visitCount * points / NUM_VISITS.

  8. If the page is unvisited and not bookmarked, or it’s a bookmarked place-query, return a 0 frecency, to hide it from the UI.

  9. If it’s bookmarked, add the bookmark bonus.

  10. If it’s also a typed page, add the typed bonus.

  11. Frecency is age_bucket_weight * (bonus / 100)

When frecency for a page is calculated

Operations that may influence the frecency score are:

  • Adding visits

  • Removing visits

  • Adding bookmarks

  • Removing bookmarks

  • Changing the url of a bookmark

Frecency is recalculated:

  • Immediately, when a new visit is added. The user expectation here is that the page appears in search results after being visited. This is also valid for any History API that allows to add visits.

  • In background on idle times, in any other case. In most cases having a temporarily stale value is not a problem, the main concern would be privacy when removing history of a page, but removing whole history will either completely remove the page or, if it’s bookmarked, it will still be relevant. In this case, when a change influencing frecency happens, the recalc_frecency database field for the page is set to 1.

Recalculation is done by the PlacesFrecencyRecalculator module. The Recalculator is notified when PlacesUtils.history.shouldStartFrecencyRecalculation value changes from false to true, that means there’s values to recalculate. A DeferredTask is armed, that will look for a user idle opportunity in the next 5 minutes, otherwise it will run when that time elapses. Once all the outdated values have been recalculated PlacesUtils.history.shouldStartFrecencyRecalculation is set back to false until the next operation invalidating a frecency. The recalculation task is also armed on the idle-daily notification.

When the task is executed, it recalculates frecency of a chunk of pages. If there are more pages left to recalculate, the task is re-armed. After frecency of a page is recalculated, its recalc_frecency field is set back to 0.

Frecency is also decayed daily during the idle-daily notification, by multiplying all the scores by a decay rate of 0.975 (half-life of 28 days). This guarantees entries not receiving new visits or bookmarks lose relevancy.

Adaptive Input History

Input History (also known as Adaptive History) is a feature that allows to find urls that the user previously picked. To do so, it associates search strings with picked urls.

Adaptive results are usually presented before frecency derived results, making them appear as having an infinite frecency.

When the user types a given string, and picks a result from the address bar, that relation is stored and increases a use_count field for the given string. The use_count field asymptotically approaches a max of 10 (the update is done as use_count * .9 + 1).

On querying, all the search strings that start with the input string are matched, a rank is calculated per each page as ROUND(MAX(use_count) * (1 + (input = :search_string)), 1), so that results perfectly matching the search string appear at the top. Results with the same rank are additionally sorted by descending frecency.

On daily idles, when frecency is decayed, also input history gets decayed, in particular the use_count field is multiplied by a decay rate of 0.975. After decaying, any entry that has a use_count < 0.975^90 (= 0.1) is removed, thus entries are removed if unused for 90 days.