Does Firefox just *close* or do you get a Firefox Crash Reporter? Also, does this happen after you re-set Firefox to its default state?

See Refresh Firefox - reset add-ons and settings.

Firefox just closes, no crash report or anything. However I have since noticed that it seems to only happen with two particular favourites. And if I have other tabs open only the tab I am using to access these sites closes. When I re-open the tab or re-open firefox those sites load fine. This seems to indicate a problem with those sites (they are business related).

Can you link me to those favorites, please?

There are actually 3 now:

• http://www.discovery.co.za/portal/
• https://www.blueprintonline.co.za/blueprintportal/index.htm
• https://mdsonline.momentum.co.za/mdsonline/welcome/loginScreen/loginScreen.jsp

Would you have any security software installed on your computer that might block these sites?

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions or if hardware acceleration or the JIT compiler is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't click the Reset button on the Safe mode start window or otherwise make changes.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes
• http://kb.mozillazine.org/Safe_Mode

You can remove all stored data from a specific domain via "Forget About This Site" in the right-click context menu of an history entry (Show All History or History sidebar).

Using "Forget About This Site" will remove everything like bookmarks, cookies, passwords, cache, history, and exceptions from that domain, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks.
You can't recover from that "forget" unless you have a backup of the affected files.

It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.

No security software for those sites. Been working fine until about a week ago.

Ok, will try all this. I regularly clear the cache and cookies, in fact my settings are set to do this every time I shut down. Will try the rest.

Just to be clear, Firefox doesn't have a "favourites" menu item. Do you mean bookmarks or do you have the PlainOld Favorites extension installed, and is what you mean when you say "favourites"?

Note that a "Reset" creates a new profile and tries to import some settings from the old profile leaving the old profile intact, including the cache folder (about:cache) that can take up to 1 GB.

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

Firefox 15+ versions will move the old profile folder to an "Old Firefox Data-##" folder on the Desktop that gets a number appended if you use reset more than once, so you can no longer use the profile manager to revert to the old profile.

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.org/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 the 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

Yup, I mean bookmarks.