A previous hint covered an AppleScript that does this in Safari --unfortunately I am hooked on Firefox, and the script won't work with it. (As noted in the comments, Firefox AppleScript support isn't that great.) But after some trial and error, I was able to write something that seems to work well for Firefox, despite being a bit of a kludge.
Copy and paste the script into Script Editor, and edit it to reflect your screen size and homepage. Then save it as a .scpt file, run it with a hot key, and get a nice sliding window in Firefox!
[robg adds: I have mirrored the script here on hints. I tested this, and it works well (at least for one window).]

