The thing I dislike about IE is when you have many Windows open, it is harder to keep track of them and each copy you have open takes up more memory. Tabbed browsing just helps me keep track of open Windows. I cant see how it could slow you down though.
I think there may even be an option not to use tabbed browsing (and if not maybe someone can write an extension)
One of the best feature in Firefox is the middle-click button - use it on a link and it opens in a new tab without setting focus on it, allowing me to continue reading the current webpage. With IE it is easier to read the whole page and then visit the linked
Mozilla may be bloatware, but I don't feel Firefox is - it is simply a browser, nothing more, nothing less. It is also easy to add features through extensions - extending IE requires ActiveX and admin rights as well as a possible reboot. IE is more bloatware
as it also includes Outlook Express and is a 25MB download. If users did not have a browser installed they may be download Firefox rather than IE, as it is a much smaller download.
Wait until Firefox 1.0 is out and then perhaps try it again - it will be more stable and probably a smaller download. It may also be deployed within a network to replace IE.
easily customise Firefox and make it look completely different, add extra company-specific toolbars and use it on many OS's.