You can't set all prefs to the default on the about:config because some are set by Firefox to specific values and making changes to prefs without knowing the consequences can be dangerous. This is why you initially get a warning page when you first open the about:config page.

Why do you want to set prefs to the default?

If you have prefs from extensions that you had installed in the past and that are no longer present then you can safely right-click those and reset them and they will be removed after a Firefox restart.

• http://kb.mozillazine.org/Resetting_preferences

Since I had this profile for a long-time and who knows what mistakes I have made in the pass : )

To go through ALL configs would be a lot of work and I'm no pro in knowing what everything means.

I'm having a little problem with memory hogging when FF has been on for some time with 15-20 windows.

I could try to make a new profile and send history bookmarks and addon to that one. That would be an alternative. But not sure how to the everything including auto-complete history to go to the next profile. But maybe that's for another question.

You can delete the prefs.js file in the Firefox Profile Folder to reset all prefs to the default values and start from scratch with making changes.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

• Help > Troubleshooting Information > Profile Directory: Show Folder

What things get affected by that? Will that change behavior of addons? Erase history?

Thanks!

This won't affect the history and the bookmarks and other data stored in an SQLite database file.

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

This will only effect the settings that you make in Options and data/settings that extensions store as a preference, so you will have to redo such settings.
You can opt to edit the prefs.js file and only remove specific lines if you wish to keep some settings, but you have to be cautious not to remove lines that Firefox uses to store specific settings needed for normal operation.
Settings from extensions that you used in the past, but no longer have installed may still show as prefs and those can safely be removed.

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

Ok, thanks! That might be something to try.

What settings do you recommend someone to change under options? I guess "block phising sites" just makes FF slower, and that kind of setting is only needed for people that don't know anything about security.

So that one I could uncheck as I currently have. Also, web cache is currently set from auto to 50 MB. If I don't set it manually it goes to 1 GB and that can hardly improve browser speed.

Any other tips also very much welcomed!

Just leave all settings in options at their default unless you know what you are doing.
Disabling security features makes you vulnerable to security issues and changing other settings can cause websites to malfunction or content may be missing or you may suffer rendering issues.

• https://support.mozilla.org/kb/Options+window

Ok, I'll start another thread regarding tweaking. Thanks a lot as usual for your feedback!