Now, one way that will make it REALLY easy is using NameMapping and Aliasing.
1) Create a Project variable called "BrowserProcessName" and set the default value to the browser of your choice (iexplore or firefox)
2) Open either IE or FF to the home page and then map the browser process as NameMapping.Sys.Browser.
3) Edit the NameMapping node for the browser and edit the ProcessName property.
4) Change the mode of the property from "Constant" to "Project Variable"
5) Select the project variable in the drop down.
Much of everything else works then from there. Then, to switch browsers mid test, all you need to do is reassign the project variable.
You might need to do some additional work with aliasing objects on pages and such since different browsers render things differently but, again, using things like Conditional mode and Required Children as well as wildcards and such you can make a very dynamic NameMapping/Alias scheme that should be flexibile enough around whatever browser you're testing.
Automated Testing Evangelists
Definition expert - noun - Unknown drip under pressure