Generally happens to me when I've had the browser open for a long time. Sometimes small bugs and cache issues happen, and the best way to fix the issue is to simply reset the computer. I don't worry about uninstalling and re-installing it anymore, just letting the computer reset for a few seconds and re-opening firefox again tends to fix the issue.
I also notice, if you leave the browser open for too long, it won't save when you reply to comments. So I tend to refresh my tabs when commenting every 5 or so comments, just to make sure it's gone through properly.