I’ve been trying to do that for years and did the most exotic things to make it happen. I’ve used four different browsers in parallel and I’ve even used a virtual PC running within my PC (that kind of defeats the desire to use less memory, but it feels neat anyway). In the end, a collegue in the office, who happens to use emacs as his main web browser :D , pointed me into the right direction.
Now I have three completely independant instances of Firefox running at the same time!
So what’s the trick?
Firefox has command line options to let you start multiple profiles and specify a certain one. In our case, we’re going to change the command line to:
“C:\Program Files\Mozilla Firefox\firefox.exe” -no-remote -P NewProfileName
To get started, check the name of your current profile. On Windows you can find it in “c:\Documents and Settings\[user-name]\Application Data\Mozilla\Firefox\Profiles”. It will generally have a few characters and numbers, a period and then the profile name (in my case it was something like “36fc232a.default”). Use this to adjust the settings of the icon you use to start up Firefox. On Windows, right-click on the icon and select “Properties”; you can add the options in the field called “Target”:
If you click on that icon now, it should start up Firefox just as before (ok, this is not the neat part yet :D ).
Now make a copy of the icon (I right-click drag it into a folder and select “Copy”) and change the command line options (and file name) again, only this time choose a different profile name. If you want to use a copy of your existing profile (with all cookies, bookmarks, themes and add-ons), you can do that by going into the folder where your profiles are stored (mentioned above) and copying your default profile. Now when you start up Firefox with that icon, it will bring the profile manager since it can’t find that new profile. Create a new profile and use the exact name you used in the options. You will then have a choice of either creating a completely new profile or using an existing profile folder.
Now you have two instances of Firefox running at the same time. They’re completely separate, so if one crashes, the other will continue normally. If one starts using too much memory, you can close it and restart it without impacting the other one. If you have conflicts with add-ons or want to use different cookie sets, just use a separate instance.
Since the various instances will generally look the same and be hard to keep apart, I just applied different themes to them. The “Safari-style” theme is my private one, the blue one is used for all my work-apps and the normal one is used for all kinds of testing.
This trick should work on all platforms with Firefox, not that I tried it out so try it at your own risk :) . Now if only I could migrate my IE profile back to Firefox …