You may have noticed that, the Firefox versions that ship for Linux, don`t have the ability to select the url in the address bar on single-click of a mouse, whereas this is set by default in Windows versions of Firefox. If, you feel this feature should be there in the linux version of firefox too, the do the following:

1) type about:config in the browser`s address bar and hit the return key(Enter Key)

2) Then click on I`ll be careful, I promise button that shows up on the next page.

3) In the field that says Filter, key in the following word: clickSelectsAll

4)You will see that two entries are shown now viz. browser.urlbar.clickSelectsAll and browser.urlbar.doubleClickSelectsAll. Setting the value of the former to true by double clicking on it will enable Single-CLick url select option and setting the value of the later to true will enable Double-CLick url select option.

5) Restart Firefox, and you are done.

PS: This works on Firefox for Windows too, if you wanna do a vice-versa