Looks interesting but my first thought is that if I want to access my gmail on someone else's computer I would need to install the firefox extension first.

It's just a bookmarklet, not an extension, and if you're not at your own computer, there's a "mobile" version that can be used from anywhere to generate the correct password.

I've memorized a few (non-genpass) passwords for sites that I regularly use from other computers -- gmail being one of them. I also use the mobile version which nuclear_eclipse mentioned.