Is this possible or has it already been done?

Ideally current tab URL (or all tab's URLs) would be opened in Firefox. Possibility to select target browser from dropdown/popup/context menu would be useful.

Note: need to be able to specify in Options <path-to>browser.exe (or multiple entries one for each browser target together with command-line options)

I use Portable Firefox usually but also have an installed version so normal Registry lookup would give incorrect results. (I'm on WinXP)