It's quite easy in firefox. Just have two firefox shortcuts, one of "firefox -P default" which you use normally (default profile) and one with "firefox --no-rem

It's quite easy in firefox. Just have two firefox shortcuts, one of "firefox -P default" which you use normally (default profile) and one with "firefox --no-remote -P" which, on launch, will give you a dialogue to select or create a named profile. In addition, the "--no-remote" bit means you can run any number of firefox instances side-by-side with no issue.