I make quite good use of the little search box on FireFox's location bar.

I've got quite a few engines that I use often and have let the other pre-installed ones sink to the bottom (although I can't bring myself to delete them outright). While writing in particular I like to use the dictionary and thesaurus at Chambers and the Online Etymology Dictionary to check up on words: whether they mean what I expect, what I could use instead, whether it's anachronistic for my characters, and so on.

So often do I want to look up a word on more than one of these (and so frequently do I only want one but get the wrong Chambers search because they have the same icon) that I'd like a way of sending the same term to all three at once. Having thought about it, I've come up with a couple of different ways of doing it:
  • A small server-side script in one of my domains to accept a search term and spawn a frameset containing results from different sites. I'd need to learn the format of query strings for the search engines I wanted to use (very easy) then I'd need to make a MyCroft plug-in so that the combined search can appear in my search box (probably not too hard). It would be fine for personal use, but not suitable to distribute because it requires a web server, which is overkill.
  • Build a FireFox extension. Make Firefox create the frameset locally and fetch the search results into it (or just open them in consecutive tabs). Probably the hardest part is maintaining MyCroft compatibility (without which it remains valuable for personal use by wouldn't be versatile enough to distribute): the extension would have to put a dummy entry into the search box, intercept submissions to it and spawn new requests to other MyCroft plugins. Or perhaps it would be easier without such tight integration: an options dialog to set the search engines used and a MyCroft plugin pointing to an 'about:'-style pseudo-address. It would be a far more valuable tool than doing it server-side, but considering how little I currently know about Firefox extensions it's perhaps not worth trying to learn what I'd need to know.
Anyone got any thoughts?

While I remember, does anyone know of a keyboard shortcut to change the search box's currently selected engine?

Edit: I was thinking of about:config when I said an 'about:'-style address, above: in fact it would be a URI beginning 'chrome:'.