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Edit in Firefox the page displayed in any tab, with formatting tools.

Press the Page Hacker button (or hit Ctrl+F2) to edit the currently viewed page, as in a text editor. A floating toolbar appears to change the text format.

It can be used to remove unnecessary content from a web page before printing it, make a hoax, or even use Firefox as a text editor by using it an a blank tab.

The changes are only seen by you and are not permanent (if you are looking for it, Greasemonkey can do that).