There are several ways that allow you to test Firefox 3 without interfering with Firefox 2 if that browser is also installed on your system.
If you simply install the regular Firefox 3 version it will use the same profile folder which means that it will check all add-ons for compatibility and Firefox 2 will do the same on the next start provided that you don't install the update in the same directory in the first place.
A practicable method is to install the portable version of Firefox 3.
The main advantage of this method is that the portable version does not interfere at all with Firefox 2 as it runs completely independent so to speak giving you all the time in the world to test the new version of Firefox before you upgrade the installed version.
This method does have a downside however, none of your settings, extensions or customizations are imported which means you cannot access your bookmarks for instance.
Since it's only installed for testing purposes it should not matter that much however and you could still move some of the files - like the bookmarks.html file - from the Firefox 2 profile folder to the one of Firefox 3 to be able to use them. The same is true for extensions and such even though it may require more time to get that right.
Another possibility is to create a new profile and assign that shortcut to Firefox 3. This still has the effect that both browsers use independent settings and don't interfere with each other. Since it is a little bit complicated to setup I would probably go for the portable version to test Firefox 3.
Lastly you can simply install Firefox 3 in another directory which would mean that it would use all of your settings but that is generally not recommended as it may modify browser settings in a way that is incompatible with the old version.
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