Last week. For some reason just over the past month or so, a couple times Google Hangouts has randomly decided it doesn't support Firefox one day, then work fine the next.
A bit further back (maybe a year or so?), I've also had issues with Youtube and my bank's website. Both of those seem to have been fixed, though.
That’s probably just Google’s aversion to paying for QA. I get the same kind of problems in Chrome and their apps which randomly go away after clearing the cache, resetting the app, etc. I suspect the root cause is that most of their developers want to work on cool things rather than fixing the previous round.
A bit further back (maybe a year or so?), I've also had issues with Youtube and my bank's website. Both of those seem to have been fixed, though.