I hope I can explain this properly. This happens when watching a flash video, either from youtube or any flash video embedded in a website. If the download bar catches up to the buffering bar the video hangs up but never resumes. When this catchup happens I lose all connectivity to the web. Other devices on my network work fine. Firefox continues to work but just gets time-outs when a new link is selected. When this happens I close Firefox, when Firefox is closed it remains active in the task manager. Even after a long wait it remains active in the task manager. If I force Firefox to close and restart it I still have no internet connectivity. I've tried using the internet connection repair in windows to no avail. The only way I can get back internet connectivity is to re-boot the computer. This happens on both an XP machine and a WIN7 laptop. Both computers have the same version of Firefox and Flash (the latest). This stated happening around Firefox 17 or so. I've been waiting, assuming a later update will fix the problem. Yet it remains, if while watching a video the buffering bar stays ahead of the download bar all is well and the computer works for hours.
Do you have the same downloader install on both machines..?
I think its a problem with the downloader add-on for firefox that you are using,
Try Firefox Safe Mode to see if the problem goes away. Safe Mode is a troubleshooting mode, which disables most add-ons.
(If you're not using it, switch to the Default theme.)
On Windows you can open Firefox 4.0+ in Safe Mode by holding the Shift key when you open the Firefox desktop or Start menu shortcut.
On Mac you can open Firefox 4.0+ in Safe Mode by holding the option key while starting Firefox.
On Linux you can open Firefox 4.0+ in Safe Mode by quitting Firefox and then going to your Terminal and running: firefox -safe-mode (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
Or open the Help menu and click on the Restart with Add-ons Disabled... menu item while Firefox is running.
Once you get the pop-up, just select "'Start in Safe Mode"