After upgrading to 16.04, when I type into a javascript text field like this search field, every keystroke erases the one before it. I've also noticed subtly different behaviour in the location bar; matching URLs are no longer pre-selected until I cursor down.
I have removed my entire ~/.mozilla directory, purged firefox* (and manually deleted all the hanging firefox files in /etc and /usr/lib).
Even after what should be a completely fresh install of Firefox 49.0 (and a short try with an early v48 build), the problem persists. The only way I can make the behaviour stop is to disable javascript in about:config.
Possibly related: when I attempted to post this question with Firefox, the post hung, and I got an error from StackExchange about an unusually high number of requests.
Anyone else seeing anything similar? What else can I try?
UPDATE:
It's more than just Firefox. All gecko-based applications seem to be misinterpreting keyboard input, as was Eclipse. I suspect keystrokes were going to the wrong windows, somehow.