Gah, why would you want to run both? Well, Firebug is the suck in Firefox 3. I've been doing a lot of javascript/ajax development lately.
When using Firefox 3 and the Firebug console, I get the following error occasionally:
Lifehacker had a nice post on how to run Firefox 2 and 3.
For most cases Firefox 3 is just fine. But for the serious javascript coder, Firebug just isn't ready for prime time in Firefox 3.
commandLine.evaluate FAILS: [Exception... "Security Manager vetoed action" nsresult: "0x80570027 (NS_ERROR_XPC_SECURITY_MANAGER_VETO)" location: "JS frame :: chrome://firebug/content/commandLine.js :: :: line 100" data: no]