Firefox only (Windows/Mac/Linux): Firefox extension gui:config adds an advanced settings panel with easy access to options normally hidden behind cryptic about:config options. Once installed, the extension adds a new Advanced Settings item to the Tools menu for accessing the hidden settings. Rather than making you install the add-on to see if the settings apply to you, we'll just take a quick tour through all of the screens so you can visually figure out if you want to install it or not.
The Accessibility tab adds in quite a few useful settings like middle-click paste or changing the backspace key's default setting to go back a page (an annoyance for many people).
The Browser tab gives you options to change the amount of cache being used or when the cache is checked, letting you cache more aggressively for performance reasons.
The Style tab lets you turn off Favicons, tooltips or disable blinking text (another annoyance).
The Downloads tab lets you change the download directory and adjust the alert settings for completed downloads.
The Bookmarks tab lets you configure the amount of bookmark backups that are created (usually once per day for 5 days).
The Developing tab is probably only useful for programmers, and most people won't want to mess with these settings.
This is the type of extension that is so useful it makes you wonder why it's not included in the main distribution—they could hide it behind an advanced menu for serious tweakers only. It should be noted that not every setting is accessible from the GUI, so for certain tweaks you'll still need to take a trip to about:config. gui:config is a free download, Firefox only.