Mouse behavior in Linux
From MozillaZine Knowledge Base
In the X Window system used on Linux and other UNIX variants, making a selection sets the primary selection, and in some applications such as terminals, clicking the middle mouse button pastes the primary selection. In Firefox and SeaMonkey for Linux, the preference "middlemouse.paste" allows similar behavior in text fields if it is set to true in about:config.
There is a separate preference for the behavior of middle click in the content area and on a browser tab. If you set the preference "middlemouse.contentLoadURL" to true (default for Firefox and SeaMonkey on Linux/Unix), middle clicking in the content area and on a browser tab will attempt to load the current primary selection as a URL. If you want middle-click on tabs to close them as it does in Windows, set this prefence to false.
Double clicking in text usually selects a "word". The preference "layout.word_select.stop_at_punctuation" determines whether punctuation such as / and - separates a word; this determines whether double clicking on something like example.com/the/path selects the whole URL or part of it. Firefox on Linux/Unix has this preference set to false since bug 190615, but it is still true in SeaMonkey for Linux (bug 611162).
The same preference also affects how far Ctrl+Left/Right arrow moves the caret in text, and how much Ctrl+Backspace erases.