Did you check your security software (firewall)?

A possible cause is security software (firewall) 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 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.

See:

• Server Not Found - Troubleshoot connection problems
• Configure firewalls so that Firefox can access the Internet

I had a similar problem. Most noticable for me is on youtube where the songs will only load about 1½ minute worth of music. Disabling my firewall increases browsing speed of webpages, and allows me to get the full song on youtube.

I tried deleting the 'instance' of firefox (and any other browsers) in my firewall application settings (even though it was set to 'allow all' there) but the problem persists whenever my firewall is enabled. Disabling allows again proper speeds and connectivity. Any ideas? I'm using 'PC Tools Firewall Plus' for my firewall.

i would .. first shut down your firewall... then see if your speed if right and make sure your not connecting thru a proxy options.....then advanced tab then network tab...then connection settings button and if you want to go back to a old firefox . just install the older 1 . i like to uninstall firefox first. then re-install . dont forget to save your bookmarks

@buzzerghost
Make sure that your Firewall allows plugin-container.exe to have access. That is what Firefox uses to run plugins like Flash.