Sometimes Mozilla Firefox closes unexpectedly and restarts by itself. Or some plugin makes Firefox crash, usually Adobe Flash. Even when just browsing, many ads use flash cookies and are stored in your Firefox temporary folder. This will make your Firefox slower.
Another thing that makes your Firefox stop responding is JavaScript. WYSIWYG editors are one of them. WordPress text editor uses WYSIWYG. The most annoying thing is when you are almost finished writing an article, Firefox crashes and you forgot to save. This happened to me twice, and I was upset that time.
So after that happened, I was looking for a text area cache at the Firefox addons website. And I found what I was looking for. You can use Textarea Cache to save anything you write inside a text area or WYSIWYG editor. Every keystroke you press will be captured and saved to a file cache. And if something happens like Firefox crash or Windows BSOD, you still have your backup, and your article is saved.
This plugin helps me so much. So if you found a similar problem to my case, then it is worth to try this plugin.