According to Mozilla's Support Site, Safe Mode does more than simply disabling all of your extensions:
Clicking the Start in Safe Mode button will temporarily disable your extensions and themes, turn off hardware acceleration and reset toolbar and button customizations. When you leave Safe Mode and start Firefox up normally, your extensions, themes, and settings will return to the state they were in before you entered Safe Mode.
Safe Mode sounds more like a temporary reset, as it disables custom settings, as well as themes and extensions. Hardware acceleration is also disabled.
Thus, you could use this knowledge to continue your troubleshooting process and find out exactly what is causing your browser to go haywire. You've already tried disabling all of your extensions, so next, try disabling any themes as well. Then maybe undo your customizations. Lastly, try disabling hardware acceleration.
Of course, another troubleshooting step might be to just create a new profile and see if the problem occurs with a fresh one. To create a profile, close Firefox completely and run the following command, clicking "Start", then "Run" then typing "cmd" to pull up a Command Prompt:
Then follow the prompts to create a new one. If the problem is resolved, you can then copy over your extensions and bookmarks to the new profile.