even for RoR or python - might make sense for those working on multiple projects simultaneously, using different generations of frameworks
Every language should have something as simple as these. Heck, it should be part of the OS, IMO; why I can't (easily) run two different versions of Firefox with two different profiles at the same time is beyond comprehension.
Unless there's some serious increases in clock speeds, I'm not sure doing it for every language is worth it. As it is, VMs are a great, general purpose system that are easy to set up, use, and replicate.
* Install your different Firefox versions in different locations (so one doesn't overwrite the other)
* Use "firefox -ProfileManager" to create at least one different 'profile', which is Firefox's user data set
* Use "firefox -P <profilename> -no-remote" to launch Firefox. -no-remote allows multiple Firefox instances to run on the same machine. -P profilename is necessary because you need a separate profile for each running instance.
More info at http://kb.mozillazine.org/Command_line_arguments
Still; mindboggling. Different version = different binary launched, which accesses a different and non-locked profile. Why the hell does it instead open a new window of the currently-running version?
As a heads-up to anyone considering this: Sync doesn't (currently) like multiple versions. 3.6.10 won't sync now that I've hooked up some 4 betas.