I opened a mailing list archive on link to marc.info last night with Firefox on my Nexus 6P (stock firmware, monthly security patches applied as they come out, unrooted). Pretty quickly after the page loaded, Firefox gave me a worrying notice, saying it needed to "restart to complete changes":
That's awfully similar to what it looks like when you install an extension and it needs a restart. And it was indeed a real Firefox dialog (it goes above the page content area, and clicking restart actually restarted Firefox).
I checked Tools → Add-ons, and I don't see any new/unexpected extensions (I have LastPass, OpenH264…, QuitNow, Save Page, Tap Translate, View Source Mobile, and uBlock Origin shown there)
Also, the site—marc.info—is a well-known mailing list archive for open source projects. I wouldn't expect them to be pulling stunts like secretly installing extensions.
My guess is I've just misunderstood what Firefox is telling me, or this is some Firefox bug (e.g., it's a normal update from addons.m.o, but it's just mis-reporting the source). But I couldn't find anything when Googling or searching here. Seems odd I'd be the only person to notice this!
What is Firefox trying to tell me?