O.k. so I'm running "Windows 7 Ultimate Edition". I recently downloaded the latest edition of Firefox but I've had Firefox for years and this has been a problem for many months. My computer is a "Compaq Presario CQ56" Laptop and I run "ESET 5 Antivirus" (always updated). Anyways, Ive removed Firefox and re-installed the application many times but it didn't help. Safe mode doesn't work either. Ive even altered my antivirus program to have Firefox as an exception but that didn't help either. When I attempt to open the Firefox browser I'll get the spinning loading icon. It will appear and spin for maybe 6 seconds and then disappear. Ill be engaged in a merry-go-round of doing this 4 or 5 times till Firefox finally opens.
Try to boot the computer in Windows Safe mode with network support (press F8 on the boot screen) as a test to see if that helps.
A possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.