I personally would not run this browser as there was a very recent controversy regarding the developers thorium browser (chromium fork) where they had some weird adult content hidden in the browser and circumcision. Granted most of it was not published packages and on more so on the github repo.
The developer's explanation was that It was a small joke because at the time he had a very small user base so put the offending art In the code but admitted In the latest post that they acted unprofessionally.
It's the same guy who built Thorium Browser, and he received some backlash about what he posted on his Thorium website. Chris Titus covered this topic recently if you guys want to know more about what happened.
I would like to point out that the version in mercury browser is esr as in 'Extended Support Release' and latest firefox esr release is 115.5.0esr and this means it's not so outdated.
Thanks for pointing that out that it's ESR. Firefox ESR was updated to 115.5 on November 21, 2023 and version 115.4 was updated on October 24, 2023 (about 4 weeks for an update). Mercury 115.4 was updated on October 5, 2023 and there hasn't been an update since then. According to the releases on GitHub (github.com/Alex313031/Mercury/releases) Mercury used to get an update every 4 weeks or once a month. That may become a problem since a single creator maintains updates for several different software packages.