Elias Torres has put together a most excellent Google Calendar Quick Add extension for Firefox. If you aren't familiar with gCal's Quick Add feature, it's in the upper left of gCal (or just hit 'q' on your keyboard), and it's great: you can create a new event simply by typing it as a sentence, such as: "dinner with Jane May 1st 6 pm." No tabbing or clicking around to chose numbers and fields. Nice.
Elias' Firefox extension does a great job of enabling this slick Quick Add ability from any other window/tab in Firefox, and so far it works flawlessly, though I have a couple of tips for you. First, make sure you've already opened gCal at least once during your browser session for this extension to work; it re-uses your session cookie instead of prompting for your password. Also, if you open this Quick Add window but decide you don't need to enter an event for one reason or another, simply hit esc to get rid of the window, as its close button doesn't work.
[via Hawk Wings]