The only "downside" to using Firefox (haven't used the full-fledged Thunderbird in quite some time) is that it won't display all pages.
I put downside in quotation marks because this isn't Mozilla's fault. Any page that is html compliant Mozilla will display. the pages it has a problem with aren't html compliant, rather IE compliant.
For example, take one of your excel spreadsheets. Used that "nifty" save as webpage feature. Throw it online, load it up in IE. Works fine. Load it up in Mozilla. Doesn't work the same. Why? Excel doesn't put out html compliant pages.
Unfortunately, you have lazy coders and designers who don't go by the html standard, only checking to see if IE's bastardized rendering will make it look correct. So this isn't the fault of Mozilla, but lazy coders and Microsoft applications.
But it's much more secure, both from a Spyware and Virus perspective. It doesn't render ActiveX pages, and believe me when I tell you that's a good thing. I haven't obtained one Trojan or Spyware when running Mozilla.
Tabbed browsing is also a thing of beauty, something I can't live without anymore. There are extensions to do everything you can think of. And Themes to customize it even more.
I use Firefox as my primary browser, only opening up IE for pages that require it. I wouldn't even think of doing it any other way.