My most useful Firefox about:config tweaks.

I have collected these over time, and felt that perhaps I should post them here. Maybe someone will find them useful but if nothing else, I can store special tweaks that I love and may need to refer back to in the future.

If you're not sure what about:config even is, then perhaps this link will help. That said, on to the show.
First, type about:config in your address bar and click the "I'll be careful, I promise!" button.

Below where it says "Config name," you'll enter a search in the filter field for that name.

Where it says, "New config name," you'll have to right click on the background, then select 
New > Boolean
then enter the "New config name."

Enable Spell Checking In All Text Fields
The default spell checking function only checks for multi-line text boxes. You can get it to spell-check for single line text box as well.

Config name: layout.spellcheckDefault
Default: 1 (spell checker for multi-lines text boxes only)

Modified values:
0 – disable the spell checker altogether
2 – enable the spell checker for all text boxes

Lower The Physical Memory Used When Minimized
This tweak is mainly for Windows users. When you minimize Firefox, it will send Firefox to your virtual memory and free up your physical memory for other programs to use.

New config name: config.trim_on_minimize
Select the value: True

Select all text when clicking in the address bar
In Windows and Mac, Firefox highlights all text when you click on the URL bar. In Linux (Ubuntu), it does not select all the text. Instead, it places the cursor at the insertion point.

Config name: browser.urlbar.clickSelectsAll
Default: (Varies by system)

Modified values:
False – place cursor at insertion point
True – select all text on click

Disable Addon Compatibility Checks
Firefox insists on only allowing addons that have upgraded when they upgrade, but most of the time it's a simple code that the addon developer hasn't changed. It's a minor fix and there is no reason not to use the addon. This allows previous addons to still work. Naturally it may not be a good choice for everyone.
This contains two settings.

New config name: extensions.checkCompatibility
Select the value: False

New config name: extensions.checkUpdateSecurity
Select the value: False

Fix youtube jumpiness
This is a tweak for session restore in Firefox (not Tab Mix Plus). Evidently Firefox saves the session every ten seconds (or more specifically 10,000 milliseconds) which is just insane. Most people do not need that kind of frequency and changing the value to every two or even five minutes helps.

Config name: browser.sessionstore.interval
Default value: 10000

Suggested modified values:
120000 - Two minutes
300000 - Five minutes

Disable blinking text.
Disables text that blinks. It does not disable blinking text in an animation.

Config name: browser.blink_allowed
Default value: True
Modified value: False

Delete a mistaken config setting

If you have accidentally typed in the wrong thing, or have found a mistake.
Right click on the mistake > Select Reset
The next time you open Firefox it will be gone.

For a bigger list of configurations and what they do, you can go here.