As with most releases there are a few security fixes in v21.0 However, the vast majority of the dedication appears to be towards the developers.

Just stay with v20 if you have a doubt.

DA

Here is a link that might be useful: Mozilla

I am currently staying with FF20.x.

That still leaves the question outstanding - can I have 20.x and 21.x on the same machine?

albert-
I believe that, if you go to install a more advanced version of Firefox, it will overwrite what you have.

This post was edited by kudzu9 on Thu, May 30, 13 at 0:07

Hello,

"When installing multiple official versions of Firefox, the main thing to keep in mind is that each different version needs to be installed in it's own unique program files folder (use the Custom Installation option on the Setup Type page of the installation routine) and each version should have its' own Profile; you also need to make sure that you always launch each version with the appropriate Profile.If you don't use the Custom option, Firefox will overwrite the prevoius installation in Program Files. If you are installing multiple versions, do not launch Firefox at the end of the installation procedure, see the section below about dealing with running multiple Profiles."

http://forums.mozillazine.org/viewtopic.php?f=23&t=2249039

This doesn't strike me as a good idea. You're taking something that should be simple (a browser installed to use) and you're making it complicated. Which instance has which bookmark, as an example, or which setting was changed where.

Pick the version you prefer and stick with it. You can turn off auto-update if you want. If you want to play with other versions, use a different PC.