I keep Chrome 10, Firefox 4 (caching to RAM), Safari, Opera and IE 8 (until my Windows 7 upgrade happens... soon? Hoping) just to try and see who breaks what and where. I also check out many of our sites on Android browser, Dolphin HD, Dolphin Mini and Skyfire (thanks Tegra 2 tablet); I intend to check it out on whatever they're calling PIE these days if I switch over to Windows Phone 7 this year. A surprising amount of automotive sites work great on Android's native browser, presuming you're sitting on WiFi and have a good processor under the hood... and can get them to stop loading whatever joke of a mobile site they may be pimping lately.
GM's Training and Global Connect sites play nice for the most part, but Lead Management Tool remains an IE only thing for the most part if I just want to pull a report.
I will say in regards to our other sites, VIN Solutions/MotoSnap works with IE8 and Chrome just fine (except for some frame strata issues with Silverlight in Chrome). Firefox 4 does pretty well, but some of the controls in emails on [SIMILAR VEHICLES] and it's associated tags when they're built into a template and auto-populate on bringing up a customer email (we send a summary of the inquired car plus 4-6 options as our first reply, plus answer their questions/offer) - those make it very annoying or impossible to easily clip out one car at a time, or sometimes to delete the table altogether thanks to Firefox wanting to let us play with the table.
vAuto works great in everything I put in front of it, though Chrome still wants to open every car in a new tab instead of a new Window, and the Save & Close button doesn't close tabs, just saves and refreshes the tab.
Aaaand... we have to keep IE8 as our default browser or ADP has a conniption fit.
That's the world of Chevy and Chrysler-Dodge-Jeep we live in.