Several times a week, Firefox (version 68.0.2+build1-0ubuntu0.18.04.1) is getting into a state where it refuses to open new tabs and instead says it has been upgraded in the background and needs to restart.

I'm pretty sure I haven't been running `apt upgrade` or anything of that sort since the Firefox instance in question was started. Is it supposed to somehow download updates from some unknown third-party source on its own, bypassing the package manager? Or have I been compromised?