With the introduction of macOS 10.15 “Catalina” Apple introduced new notarization requirements that all software must be signed and notarized centrally.
Whilst geckodriver is technically both signed and notarized, the way we package geckodriver on macOS means the notarization is lost. Mozilla considers this a known bug with the geckodriver 0.26.0 release and are taking steps to resolve this. You can track the progress in bug 1588081.
There are some mitigating circumstances:
Verification problems only occur when other notarized programs, such as a web browser, downloads the software from the internet.
Arbitrary software downloaded through other means, such as curl(1) is not affected by this change.
In other words, if your method for fetching geckodriver on macOS is through the GitHub web UI using a web browser, the program will not be able to run unless you manually disable the quarantine check (explained below). If downloading geckodriver via other means than a macOS notarized program, you should not be affected.
To bypass the notarization requirement on macOS if you have downloaded the geckodriver .tar.gz via a web browser, you can run the following command in a terminal:
% xattr -r -d com.apple.quarantine geckodriver
A problem with notarization will manifest itself through a security dialogue appearing, explaining that the source of the program is not trusted.