Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Set the pref browser.tabs.closeWindowWithLastTab to false on the about:config page to prevent closing the last tab from closing the browser and make a close button appear if only one tab is open.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.
You can use the Filter bar at to top of the about:config page to locate a pref more easily.

that pref does not appear in the list. How do I add a line? plugins.update.url;https://www.mozilla.com/%LOCALE%/plugincheck/ pref.browser.homepage.disable_button.current_page;false pref.privacy.disable_button.view_cookies;false pref.privacy.disable_button.view_passwords;false pref.privacy.disable_button.view_passwords_exceptions;false prefs.converted-to-utf8;false print.always_cache_old_pres;false

Disable hardware acceleration fixed the problem for me.

Oh, no it did not! It fixed it till the computer was switched off. Then the problem is back but as soon as a new tab is selected the x button appears on all tabs even if firefox is quite and started again!

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

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

See:

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