Did you know that you can actually edit pages within Firefox! You can edit the text, apply simple formatting changes and re-size and move images, tables and other objects. All you need to do is turn on the 'Design Mode' of the document in the browser. Unfortunately, there is no way of doing that directly in Firefox. That's where this add-on comes in.
Select Tools->Toggle Design Mode to turn on the Design Mode of the document in the currently selected tab. If you would like to have an icon on the main toolbar to do the same, select View->Toolbars->Customize, and drag the pencil icon from the list into the toolbar. Then you can simply click on the icon to toggle Design Mode for a document. When the design mode is turned on, you can edit text, and re-size, move, or remove images, tables and other elements on absolutely any web page. Use the toolbar to make simple formatting changes. If you're not familiar with HTML and are looking to make a quick edit to a web page, or to a template you've downloaded, you can now do that without having to take the pain of going through the source or opening the file in another editor (if you have one).
IMPORTANT NOTE: When the Design Mode of a page is on, i.e. when it is being edited, you should see an editing cursor (the blinking line) at the place where you are typing (just like in Word Pad, or any text editor). I've noticed that the cursor doesn't show up sometimes. If you do not see the cursor, try clicking on some text in the page. If it still does not show, select another tab, and then select the tab you were editing again. Now the cursor should show up.
Once you are done editing the the page, click on the menu item, or the toolbar button again, and the Design Mode will be turned off. Then you can save the page as usual by selecting File->Save As. At any moment, if you reload the page, the changes you have made will vanish (though they will still be present in the saved file, if you have saved the page). You can edit many tabs at the same time, and independently i.e. the Design Mode can be toggled on/off independently for each one of them.
PS: This add-on is also featured on:
UPDATE 07-16-10: As promised, I've added a toolbar with some basic formatting options. More updates coming soon!