When developing websites, I’ve found it incredibly useful to be able to run, or install, multiple versions of Firefox on the same computer, side-by-side, simultaneously, however you want to call it. When it comes to testing, it’s always sensible to see how things are going to be displaying in future browsers, but even more than that, when testing different permissions and profiles on a website, having multiple, separate browsers can really simplify matters.
I run all versions in 64bit mode as it is more secure, stable and snappier with the following details:
Firefox – Legacy (3.6.28 – 32bit)
"%ProgramFiles(x86)%\Mozilla Firefox Legacy\firefox.exe" -no-remote -P "legacy"
Firefox – Stable
"%ProgramFiles%\Mozilla Firefox\firefox.exe" -no-remote -P "stable"
Firefox – Beta
"%ProgramFiles%\Mozilla Firefox Beta\firefox.exe" -no-remote -P "beta"
Firefox – Developer
"%ProgramFiles%\Firefox Developer Edition\firefox.exe" -no-remote -P "dev"
Firefox – Nightly
"%ProgramFiles%\Mozilla Firefox Nightly\firefox.exe" -no-remote -P "nightly"
Personally I go for the 64bit version because it’s the right thing to do. I also find it “feels better” when there’s a load of tabs open.
I also store all the profiles in a single location in Dropbox so I can use this setup on other machines, or even with the Portable Apps versions.