Are you facing firefox browser's constantly lagging and not responding problem? After upgrading to newer version 5.0, i was facing that problem very often. When i try to open new tab, it automatically stops responding and does nothing for more than minute. I thought it's because of hyper threading, i tried to lower that and I have also tried clearing my cache and cookies. The problem was not in my computer, it was only on firefox browser. So i did know there is something wrong in firefox. Then i downgraded it to older version 3.6  but that problem was still there.

Actually the problem was not in newest version, there were some firefox extensions making firefox hangs. I tried to remove all plugins one by one then checked it. Finally i found that extensions, that was the ant video downloader causing the problem. Actually that was installed in firefox to download videos from facebook, youtube and other sites. Finally that solved my problem. But there are also other plugins like Java, Adobe Reader, or Flas, which also make firefox hangs.

There was any funny event i used to face every time, after getting hangs there was only one option, firefox shut down. But when i tried to open it again, it used to say "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system." The most common cause of this error is that your Mozilla application did not shut down properly. It might still be running in the background (even though it's not visible) or it might have closed abnormally, without deleting the lock file. At that time you have to open task manager and stop the process firefox.exe. That was really funny for me.

I've found many people are getting confusion on firefox hangs and crash, when a program hangs, it no longer accepts user input and does not seem to be performing any actions; a crash ends the program. Other terms for hanging are "freezing" or "not responding".   A hanging program does not respond to your actions. In other situations, Firefox responds to your menu and button actions but fails to load any websites. I've collected some reasons of firefox hanging, which may help you avoid not responding problem in the future. please see list below:

- Hyper-threading - Prolonged or extensive use
- Deleting many bookmarks and multiple levels of sub-folders - Downloading many files or saving images
- loading sites with javascript/flash/PDF files
- during exit to remain history in memory
- opening the History menu or the History sidebar
- loading Automatic proxy configuration file
- opening multiple copies of its Session Restore file
- plugins like Java, Adobe Reader, or Flash - Spybot S&D's "Immunize" feature
- SwitchProxy Tool
- Extensions
- Live bookmarks
- Internet Security Software

I found these list for making firefox hangs, i hope now you will get some idea why your firefox is getting that problem frequently. You can get more information on official site of firefox. If you have noticed any other issues please mention in the comments.