Hello Vibeke. It's possible that you are having a problem with some Firefox add-on that is hindering your Firefox's normal behavior. Have you tried disabling all add-ons (just to check), to see if Firefox goes back to normal? Whenever you have a problem with Firefox, whatever it is, you should make sure it's not caused by one (or more than one) of your installed add-ons, be it an extension, a theme or a plugin. To do that easily and cleanly, run Firefox in [http://support.mozilla.com/en-US/kb/Safe+Mode safe mode] (don't forget to select ''Disable all add-ons'' when you start safe mode). If the problem disappears, you know it's from an add-on. Disable them all in normal mode, and enable them one at a time until you find the source of the problem. See [http://support.mozilla.com/en-US/kb/Troubleshooting+extensions+and+themes this article] for information about troubleshooting extensions and theme and [https://support.mozilla.com/en-US/kb/Troubleshooting+plugins this one] for plugins. If you need support for one of your add-ons, you'll have to contact its author.

See also https://bugzilla.mozilla.org/show_bug.cgi?id=542695

I've been having the same issue ever since I installed the Roboform add on.