Librewolf is based on Firefox. Even if you uninstall Firefox (not sure if you have done so already), and for some reason you close Librewolf, some times the process takes a good amount of time to really get killed. The same happens to Firefox, and I would consider it a Firefox upstream bug. I don’t know under which conditions that happens (when I’ve experienced that, it seems sort of random to me). That said, when that happens, even though Librewolf is what still has some process alive, the message you’ll get is that Firefox is still running (it’s just a problem on the message, but it’s produced by Librewolf). It might be it’s not worth changing the code around that message for Librewolf, who knows… That might be filed as a bug to Librewolf, though I’d say the most important one is on upstream Firefox, since at times it takes a pretty long time to really close. Actually that’s also inherited on Thunderbird, at times you close it, and the thing keeps processes alive for quite some time, and if you don’t wait enough, and start thunderbird again too early, it’s the same thing, you get a similar message for Thunderbird. And of course, it also happens to Firefox itself, :)

So I see two issues at once, but the major problem is on Firefox, and I’d guess the upstream one is hard to catch…

seeking feedback/resolution.