With the Firefox Download Day almost over, there is likely to be a whole load of new Firefox users as of today.
Even if you are a regular Firefox user, you still may not know about the advanced configuration options. To get these options, type about:config into the Firefox address bar. Once you hit return, you get a warning because modifying these preferences can (in rare circumstances) cause strange behaviour.
Clicking "I'll be careful, I promise!" brings up a huge list of preferences, most of which aren't accessible from the normal Preferences dialog. As the list is so long, the best way to find things in it is to start typing the name of the preference into the "Filter" box at the top. Once you have found the one you want, double-clicking it allows you to change it. Here's a list of my top 12 hidden Firefox preferences. All of them work in Firefox 3, but only some work in Firefox 2.
The default behaviour, true, means that when you click the address bar, it automaticall selects all. Changing this to false stops this, and just inserts the cursor where you clicked.
By default, Firefox displays a close button on every tab. Setting this preference to 0 will only display a close button on the currently open tab. Setting it to 3 will move the close button to the far right end of the tab bar, and setting it to 2 will get rid of all close buttons.
Double-clicking this will set it to true. This makes searches from the box in the top right open in a new tab rather than the current one.
Determines the maximum number of results that drop down from the URL bar when you start typing. Default is 12.
Allows you to change the suffix that is automatically added when you don't specify one. For example you might want to change this to .co.uk so you will be directed here if you only type www.macosxtips.
If you open a whole folder of bookmarks at once using "Open All in Tabs", they replace all of your open tabs. Setting this preference to false will cause the new tabs to only replace the current tab and add in the rest after it.
Setting to 1 Makes the Find function (Command-F) case sensitive.
Set this to true to enable the click-and-hold way to open contextual menus.
If you open lots of tabs in Firefox, they shrink down so they all fit across the screen. After a certain limit, your tabs don't get any smaller and you have to scroll to the left and right to view other tabs. This preference allows you to set the minimum width. Default is 100, lower values will give smaller tabs.
Setting this to false disables the yellow tooltips that appear when you mouse over the bookmarks bar.
Changing this value to 2 makes the spell checker work in all text boxes, not just the large text fields.
This changes the behaviour of the backspace key. setting it to 1 makes the backspace key scroll up a page in the current window. Shift-Backspace will scroll down. 2 to make the backspace key do nothing. Except delete when text is selected, of course. Changing to 0 will change back to the default behaviour of going back a page.
Of course there are hundreds more hidden settings in this list, including ones for extensions you have installed. If you know any more, add them in the comments.