Install the Win64 version in a different folder from where the 32-bit version is installed, using the Custom option. And if you want to be able to run both versions at the same time create a new Profile for Win64.

If I have the two versions installed in different folders can I run them alternatively - one after the other multiple times (not at the same time) on regular bases with the same profile so I can keep all my other stuff synchronized between the two version?

It is best to keep separate profiles for different Firefox versions to avoid issues. Firefox stores the path to the last version that uses the profile and you would get a compatibility check and profile initialization after each switch.

• https://support.mozilla.org/kb/Managing+profiles
• http://kb.mozillazine.org/Profile_Manager

That's a real pity. I've switched to the 64bit version because I had issues with the 32bit one reaching the memory limit for a 32bit application - yes, I'm a heavy user and FF isn't always playing nice when left to run for a few days (weeks). Sadly the 64bit version doesn't support the Java plugin / applet and I'm almost left without a real solution ... but I guess that's the reality. At least is there any hope to have Java working in the 64bit version soon ... or at all?

The 64 bit Firefox versions only support Flash and Silverlight and that is not likely to get changed.

• http://www.ghacks.net/2015/11/25/firefox-64-bit-to-support-microsoft-silverlight-after-all/