Ok, first of all, that's just slightly ridiculous. (on their part I mean)

I would recommend keeping Firefox5 and using it for everything else except for that. For that use IE.

Downgrading to Firefox 3.0.13 is basically throwing out all the security patches between 3.0.13 and 3.6.18, and I'm not going to recommend that you use 3.0.13 to surf the web in general at this point. You can; it's possible... but I wouldn't recommend it.

I would recommend sending them an email complaining. They are doing digital bookkeeping and taxes, there is absolutely no reason that they should be encouraging you to use an outdated (3.0.13 is 3-ish years old, I think?) browser which may increase your security risk. I'm not saying Firefox is particularly insecure; I'd say the same thing no matter what browser they were suggesting you use that old of a version of. It's just not smart on their part.

I guess, another option, if you are somewhat comfortable with computers, would be to have both Firefox5 and Firefox 3.0.13 installed. There is some more info about that here https://support.mozilla.com/en-US/questions/837467

I agree, it's kind of ridiculous, but it's been that kind of a day...our email server was down, we've got too many users on our other software and I can't get on, AND we had a fire alarm that sent everyone in the building out into the parking lot for 1/2 hour...thankfully our local fire department responded and confirmed there was no fire, but we did smell something close to the odor of burnt popcorn, probably just someone's microwave lunch fiasco. Thank you for the advice - have a great day!