On the one hand, sure. On the other hand, if there hasn’t been even a tiny bug fix or feature update in that long it calls into question (at least for me) whether when there is inevitably a breaking change, security issue with a library, whatever - that it will be addressed. If I don’t have some level of confidence in that, I’d rather not rely on the tool.
This kind of concern could be handled by contacting the developer or engaging with the community around the tool to see what the project status is, and why it isn’t being updated.
Sadly no, ever web app company definitely doesn’t test under Firefox. I’m at the point where I use Firefox for general web browsing and Chromium for most web apps.