Firefox has settings controlling whether it remembers the page you were on and whether it keeps you logged in. I found this page describing how to edit the first setting: https://support.mozilla.org/en-US/kb/how-do-i-restore-my-tabs-last-time (The article is about how to turn it on, but you'd go to the same place to turn it off.) You might have to poke around to find the setting that keeps you logged in every time.

If your Firefox got automatically updated at some point, it's possible that's when the settings got messed up too?

As for the forums issue, it is one we are aware and working on. There is a bug report here if you'd like to add your report: Forum posts not appearing after posting them