You can't it will installed based on your O/S version. You can't install the same FF version regardless of the bit version it will when it update revert to the version matching your O/S bit version. As to the creators of the addon and what they choose to do that is up to them and the users of them. FF has no control what a developer does to their addon/extension.

Regardless of what WestEnd claims above you can have multiple Firefox versions whether on Windows , Mac OSX or Linux. WestEnd the OP made no mention about Extensions as it was Plugins.

The basics since forever has been to have them in separate folders and to use separate Profiles for each version and not share. For Windows use the full setup and not the online stub installer. Though Windows users could use portable Firefox for one of the two builds. https://portableapps.com/apps/internet/firefox_portable https://portableapps.com/apps/internet/firefox-portable-legacy-52

For NPAPI Plugins starting at Release 52.0 and later (including the 61.0.1 Release and current 60.1.0esr) have only allowed the Flash Player Plugin from Adobe to run. The legacy 52 ESR still allows other NPAPI Plugins to run. Windows users needs the 32-bit version as the 64-bit (Win64) only allows the Flash Player and Silverlight Plugins to run.

For Windows, the Win64 Firefox since first Release (42.0) only allowed the Flash Player Plugin to run. As of 43.0 till 51.0 Release the Win64 Firefox allowed the Silverlight Plugin to run also.

However the legacy 52 ESR will be EOL on Sept 5 with no more security/stability updates after as the current Releases will then be 62.0 Release and 60.2.0esr.

https://support.mozilla.org/en-US/kb/npapi-plugins

Not sure what other NPAPI Plugins you need to use as Oracle is discontinuing their vulnerable Java Plugin as Java 9 does not have it. Only IE and the legacy Firefox 52 ESR even allows the Java plugin to run (in web browser) now days anyways as Chromium, Chrome, Opera dropped NPAPI Plugins support back in Sept 2015.

James,

 Don't worry, I knew Westend was wrong.  Obviously, since I 

• do* have FF 32-bit and FF 64-bit installed and running; or else I

wouldn't be experiencing the problem I asked about.

 The plug-in issue is an entirely different thing.  What really
bugged me about it was that they removed support for all 

plugins other than flash and Silverlight. Seems like Mozilla

- which at one point sued Microsoft for "anti competitive
practices" has now gone over to the dark side - they should
have continued support for *ALL* plugins or else done away
with all of them - no matter whose they were.  Doesn't their
choice sort of place Adobe and Microsoft on a pedestal 

compared to everyone else?

 I'm not sure your response actually addressed the crux of the 

problem. I have both 32 and 64-bit versions installed; but it doesn't seem that there's a way of differentiating them *as far

as Windows default app selection* goes.  FF is showing as the 

default browser app; but it's apparently the 64 bit version, not the 32 bit version, and I so far haven't found a way to change

that.

 Again as far as the plugins go; I actually work for Oracle; so
I know about Java; but there are still apps I use that require
the Java plugin to run.  At least for now.  No doubt most of 

them will be updated at some point; but that hasn't happened

as yet.