So, without further ado, I present a Google bookmarklet for Mozilla: Google.
How do you use the darned thing, I hear you ask? Well, right click on the link, choose "Bookmark this link" from the pop-up menu and place the bookmark in your "Personal Toolbar Folder".
Now, if you click the Google-button in the toolbar, a Window pops up for you to enter words to search for via Google - but if you have selected some text in the webpage and click, a Google search for those words are initiated instead.
The technique (ahem, hack, you say? :-)) has been applied to all kinds of things (whois lookups, validators, bloggers, what-have you).
I still don't know if it's useful, and I don't really know what other buttons I might like, so I'm starting out soft with the Google one.
Here's one for looking up phone-numbers at De Gule Sider in Denmark - Update: This is a new version of the bookmarklet, if you enter a number it looks that up, but if you enter a name it looks the name up: Tlf
You know the tedious routine when a web-address is on a webpage, but they didn't take the small time and effort to make it a real link? You have to select the address, copy it, select the location-widget, clear the old address and paste the new one and then press enter or click... Yawn! Select the address, click Goto - does that sound easier? Here you Goto! (Update: I've updated Goto to put "http://" in front of the selected text if need be).