I had a few problems recently and tried this;
Click on the Firefox 'tab' on the top left of the page.
When the drop down menu appears, wave the cursor on "help".
Another drop down menu should appear, click on "troubleshooting Information".
On the right of the page that comes up in a new tab, there is a box marked "Reset Firefox".
If you click this box, it will reset Firefox to its default settings rather than having to go through the hassle of uninstalling/reinstalling. This didn't solve my problem (it turned out to be nothing to do with Firefox) but it did keep my bookmarks and folders bar. I did have to go back and log into everything again though.
Please don't hold it against me if you try it and it doesn't work out the same way. I am a complete technoklutz and may just have fluked my result.