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.
Interesting that security software is the problem. I had the same security software for the past 4 years with earlier versions of Firefox and it never caused a problem. I find it odd that now all of a sudden it's a problem. Sorry, but I don't buy it.
Neither the disabling of hardware acceleration or resetting of security worked. It simply takes too much time to load pages not related to what I've requested. I ask for my email and it loads a background page on sports, etc.