tl;dr: Mozilla is freezing the main portion of Firefox's user agent version at rv:109 from version 110 to 119 to avoid triggering an issue with websites checking for IE11 by checking for rv:11 instead of rv:11.0 in the user agent string. Firefox's actual version will be included towards the end of the string for the affected versions. Proper versioning will be restored with the 120.0 release in Nov 2023.

Fun fact: This is similar to how Microsoft skipped Windows 9 to avoid apps thinking they were running on Windows 95/98 and running into issues.