Firefox has the ability to detect new versions of itself. It will then download this, run the update install then then prompt you, asking if you want to restart it now or later.

It sounds like this download failed for some reason and now its in a loop. NB This process has already worked fine on all three of the PCs I regularly use so I don't think its a general problem.

If you are using a work pc, they may have changed a policy so it now blocks software from being downloaded, in which case you'll have to copy the install to your pc using a CD or pen drive - I had to do this for a colleague only this morning. Go to the Firefox Homepage on another pc & download a fresh copy to install.

Alternatively it could be a blockage from your AV software - I'm not familiar with it so its difficult to say. Again go to the Firefox Homepage using IE & download a fresh copy to install.

If that doesn't work then uninstall the current Firefox & install the fresh one from scratch. You shouldn't loose any settings in the process.