You may be able to upgrade to a new version Firefox 10.0.1 expected to be available from Friday 10thFeb. That may solve your problem. Obvioulsy you may need to download using another browser that does work, such as IE8, if Firefox is crashed or unusable.

I am presuming you are having problems that the new release will fix. If you wish to investigate further now, please look at the article

• firefox crashes
• the article explains how to obtain CrashIDs please post a couple of recent Crash IDs, that may help us to work out the cause of the crashes.
• if you only get hangs; see instead: firefox hangs
  • you may find disabling your JavaPlugin resolves the hangs issue

If you need working version of firefox in the meantime, the easiest solution may be to download, install and use firefox portable 3.6.26 as an additional browser

• see http://portableapps.com/news/2012-02-01_-_firefox_portable_3.6.26_released

Hopefully someone will update the article latest firefox issues to explain what is happening.

(Rather less user friendly technical discussions in threads:

• https://support.mozilla.org/en-US/forums/contributors/708122?
• and https://support.mozilla.org/en-US/forums/contributors/708145 )

When you say "Now I cannot use Firefox to access any web sites" could you describe the exact problem or error? Depending on the problem there are possible fixes here:

• Latest Firefox issues
• Websites don't load - troubleshoot and fix error messages
• Fix problems connecting to websites after updating Firefox
• Firefox won't start - find solutions
• Firefox hangs or is not responding - How to fix
• Troubleshoot Firefox crashes (closing or quitting unexpectedly)

Every web page I try to visit displays the same error:

Unable to connect - Firefox can't establish a connection to the server at ....

The site could be temporarily unavailable or too busy. Try again in a few

   moments.
    >> All of these sites are available using IE.

If you are unable to load any pages, check your computer's network

   connection.
   >> Obviously, my computer's network connection is fine, since I'm posting here.

If your computer or network is protected by a firewall or proxy, make sure

   that Firefox is permitted to access the Web.
   >> I had no problems prior to the automatic update to V10, so I'd be suprirsed if this was the problem.

A common cause of that sort of problem is that security software such as a firewall or even a router itself is blocking access, often because the software recognises the original version as a trusted and genuine browser, but does not recognise the new version of Firefox and so blocks it.

Have a look at your firewall/security settings, and the documentation of the software.

What security software (firewall) are you using?

See these articles for more information on the "unable to connect" error after updating Firefox and configuring software firewalls to allow Firefox to connect to the Internet:

• Fix problems connecting to websites after updating Firefox
• Configure firewalls so that Firefox can access the Internet