32-bit programs can run on 64-bit machines as most devices are compatible with lesser things.

But a 64-bit program may not run on a 32-bit machine.

Firefox is available in both 32-bit and 64-bit versions and both run on 64-bit Windows 7, 8, 8.1 and 10.

If your older 32-bit software won't run on 64-bit Windows, it may be because Microsoft no longer supplies some system files. You can try using compatibility settings to see whether that helps, or check with the vendor.

Firefox has been available as 64-bit (Win64) for 64-bit Windows ever since Firefox 42.0 Release and is listed at www.mozilla.org/firefox/all/

The Thunderbird community that makes Thunderbird (not Mozilla) has only made 32-bit version for Windows as there is no Win64 version and is actually not really needed. While 64-bit Thunderbird is actually needed for Mac and 64-bit Linux.

If you see WOW32 in a useragent then it means it is a 32-bit application running on 64-bit Windows. https://en.wikipedia.org/wiki/WoW64

Some older legacy software and games may only work or be made to work on Windows 7 and perhaps 8 (8.1) but have issues or not work on Windows 10.

Thanks for the answers - basically re the software I want to run, I know it is 32-bit with some odd calls from 16-bit days. On 32-bit Win 10 it actually runs relatively successfully on NTVDM.exe not all calls work, but I can work around them. This software has never run on 64-bit and Windows 10 pro (that I have) 64-bit does not have the capability to run back to older versions of Windows that are not themselves 64 bit

The product itself is not upgradeable beyond what I have

So: thanks for all the answers but I will have to live with 2 machines or drop the legacy software