You can set the dom.disable_beforeunload pref to true on the about:config page to disable this alert.

dom.disable_beforeunload = true The only choices for full screen permissions are "Always Ask" and "Block". You can manage the full screen permission via "Tools > Page Info > Permissions" or via the about:permissions page.

You can toggle this pref that is aimed at developers to false if you want to suppress this dialog.

full-screen-api.approval-required = false You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

http://kb.mozillazine.org/about:config

Try to rename (or delete) the xulstore.json file in the Firefox profile folder.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can try to open a new window to to see if that works differently.

Hi , thank you both for answering. I first tried to rename xulstore.sjon No effect. I see a new xul being made. Then I tried to change the dom value to true, nothing changed. Also changing the full screen option in the permissions page did have no effect.

I found a partial solution myself so that the opened tabs are at least not hidden, when I open Firefox. By right clicking in the right top corner tabs bar and uncheck Hide Toolbars but whenever I close and reopen Firefox it is nevertheless in full screen mode.

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

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

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

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

Creating a new profile was something I wanted to avoid. But encouraged by you I did it and it helped in a windows installation (mostly I work with Archbang linux). One for one I reinstalled my extensions except three: stumble, pocket and evernote and till now everything is fine.