How to install the multiple version of Firefox here i show you some great thing who can help you to setup fx multiple version in your own system.
you need to know that when you run the setup he just override the installed Firefox so you need to install the Fx in two different directories by making a custom installation.
plug-in incompatibility is a big trouble user found and a another things is that Firefox every time check for computable version when another version of Firefox you trying to run who not match which version of Firefox you run last time. for troubleshooting both kind of problem you need to setup profile for every version of Firefox.
for setting profile you need to run firefox.exe –P after closing all instance of Firefox. by running this you found the windows who help you to make new profile. in the windows you find a existing profile default who you currently have.
after making a profile you need to attach the profile with Firefox seprately then their is no problem come when two version of Firefox you run in your system. now i show you you how to attach the profile in Firefox
open the firefox shortcut properties and put their
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -P "Firefox 4"
C:\Program Files (x86)\Mozilla Firefox\firefox.exe" is a url of executable and -P "Firefox 4" is argument who run the Firefox in a profile we want to use. in this example i put "Firefox 4" because profile name is Firefox 4.
using this trick we can use multiple version of Firefox with no problem of plug-in incompatible