It could by your Anti Virus software scanning files for permissions after detecting changes in the updated Firefox. Some antivirus software may do intensive scans of each of these files during startup and block access until those scans complete.
Have a look at Firefox home page setting - tools - options - general - change it to default and see if that makes any dfference.
Have a look to see if you have the "don't load tabs" until selected option checked - uncheck this so that the pages don't load until you select them - have a look at this article.
Another problem could be your plug-ins - check them here.