User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:67.0) Gecko/20100101 Firefox/67.0
Steps to reproduce:
Upgrade from Ubuntu 16.04 to 18.04 with Firefox 67.0 installed.
Unfortunately, Firefox was built for 16.04 after 18.04.
Since the 18.04 Firefox was built a few minutes before the 16.04 version, Firefox wouldn't start because it was "older", even though both where 67.0.
LastVersion=67.0_20190517140819/20190517140819 is the version.
Checking anything after the _ is just wrong. If you have breaking incompatible changes within the same minor revision, something else is wrong.
Firefox should check that both versions were 67.0 and launch right up.
To work around I edited the file and put 66.0_xxx there instead so I didn't have to make a new profile.