See:

• http://kb.mozillazine.org/Browser_will_not_start_up

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 and the updater process.

See:

• https://support.mozilla.com/kb/Server+not+found
• https://support.mozilla.com/kb/Firewalls

Thanks- tried this but didn't help. Then found that my wife's profile had updated & is running FF8! My daughter's also updated today. It is just in my profile that nothing will load & the CPU use goes up to 50+%

It is possible that there is a problem with the file(s) that store the extensions registry.
Delete the files extensions.* (e.g. extensions.sqlite, extensions.ini, extensions.cache, extensions.rdf) and compatibility.ini in the Firefox profile folder to reset the extensions registry. New files will be created when required.

See "Corrupt extension files":

• http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
• https://support.mozilla.com/kb/Unable+to+install+add-ons

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the Tools button at the left side of the Search Bar (or click the "Find Updates" button in older Firefox versions) to do a compatibility check or see if there is a compatibility update available.

Create a new profile as a test to check if your current profile is causing the problems.

See "Basic Troubleshooting: Make a new profile":

• https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile, but be careful not to copy corrupted files.

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox