Oh, and I tried fixing it by checking my privacy settings and add-ons. Didn't work... :(

Hi  ! You've already checked your add-ons and some settings (good thinking  ! ), but still :
Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

Separate Issue; Your System Details shows;

Installed Plug-ins

Shockwave Flash 23.0 r0 Shockwave Flash 11.8 r800
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Flash: Grab the uninstaller from here: Uninstall Flash Player | Windows Uninstall Flash Player | Mac

Then reinstall the latest version.

Note: Windows users should download the active_x for Internet Explorer. and the plugin for Plugin-based browsers (like Firefox).

Note: Windows 8 and Windows 10 have built-in flash players and Adobe will cause a conflict. Install the plugin only. Not the active_x.

Flash Player Version: 23.0.0. 207 Flash Player (Linux) Version 11.2. 202.644

https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download. <Windows Only>

https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download. <Windows Only>

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Well, problem solved! I opened firefox in safe mode, and everything worked, so I disables all add-ons and started enabling the manually. It was the InstantFox add-on that caused the problems. Thank you Happy112, & FredMcD. :)

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.

Also please contact the add-on's support site to let them know.

Another problem with the address line, it does not offer me to switch to tab. Any idea how to fix this? Thanks!

naal said

Another problem with the address line, it does not offer me to switch to tab. Any idea how to fix this? Thanks!

Are you saying that above the address bar, on the left side, you don't see an <big>X</big> that you can click, to open a new tab   ?

Yeah, I can see I didn't explained myself correctly...

When there are a few open tabs (let's say gmail, yahoo and aol), if you type an address that is open in another tab (if I'm in gmail and will type "yah") it should offer to switch to that tab (yahoo). But it does not, even in safe mode.

Did you check the location bar settings?

The location bar drop-down list can show history items, bookmarked pages, pages opened in tabs, and search suggestions. You can set what suggestions show when you enter an URL in the location/address bar bar via Options/Preferences -> Privacy

• Location Bar: When using the location bar, suggest:
  History
  Bookmarks
  Open tabs

Yes, I did. :) But it didn't help. I tried it also in safe mode, but it didn't help either. :(