You can do one of two things:

1. create a separate Mac user for your wife (or you) on the computer. http://kb.iu.edu/data/akmk.html
2. Create different Firefox profiles for each of you. Profile Manager - Create, remove or switch Firefox profiles

Firefox has the concept of profiles, which are separate settings folders storing bookmarks, add-ons, history, cookies, etc. You can set up separate profiles and set up two different Firefox shortcuts to make it easy to start up in one of the two profiles.

For best interoperability, you would want to close out of one person's Firefox session completely before starting the other person's session. (You can start up using a special switch to isolate two sessions, but other programs might not be able to launch links in the correct session when running this way.)

Is this going in the right direction? There probably are "how to" articles out there on the web, but there are some Knowledgebase articles you can consult if you want to tackle it on your own:

• To create a new profile and to start up Firefox in a specific profile: Profile Manager - Create, remove or switch Firefox profiles. Note: I recommend using the default file locations suggested by Firefox, and never deleting a profile without backup up all of your profiles, in case something goes terribly wrong.

• Command line option to start up Firefox in a specific profile: Command Line Options on MDN

Well, Mac profiles will not work for us. It takes time switching between Mac profiles. I tried Profile manager, thank you for the advice. I cant really say I'm happy with it. I did set up couple of profiles and I've got another Firefox icon at startup, but when you open both you cant really see which one is which.

when you open both you cant really see which one is which

Does MacOSX let you rename the icons? For the appearance after start-up, try a lightweight theme AKA a persona: http://www.getpersonas.com/