I have a saved fdminst 3.9.7 installation file, build 1638, which I believe installed Firefox extension 3.03. But my Firefox actually has extension 3.06. I vaguely remember being asked to do some kind of upgrade in the past. Do I actually have 3.9.7 build 1641 installed? Thanks.

It's most probable that your Firefox automatically updated it's FDM extension. But why don't you check your FDM build? If you have only FDM installer, you can unpack it with innounp (Inno Setup unpacker), which is available on Sourceforge site.

OK I do have build 1638 installed. I was under the impression that this installed Firefox extension 3.03 but I have 3.06 dated Nov 2017. Maybe I'm wrong and 1638 installed 3.06 but my old notes say otherwise. When I examine the xpi file it says it is an official extension built by Mozilla but when I go to the Mozilla addon page (Free Download Manager official extension by freedownloadmanager.org) the latest version is 3.057 Oct 15, 2021. So where did 3.06 come from? Was it really part of build 1638?

A second question that may make the above moot. Should I go ahead and install build 1641? I just don't want to risk losing the functionality I have now as I use it every day.

Thank you
Richard

In my archive I have only:
* 3.0.6 extension updated automatically by Firefox - it's created on 2017-11-27.
*FDM 1638 installer with 3.0.3 extension. The certificate of the FDM installer certificate have timestamp 2017-10-27, so it's definitely older.
However, there might be silent installer update with newer certificate timestamp. Whatever it was, that way the extension could be updated only to 3.0.11 maximum. Those extension builds are signed with certificate which already expired so Firefox may warn you or block the extension when you try to install it.

If you want to use newer FDM extension (fdm-ffext2) with FDM 3.9.7, you should first remove the old extension from Firefox, install FDM build 1641 and only then go to addons.mozilla.org to download a new extension. Works for me.

I did what you suggested and it worked perfectly. Thank you very much! Now I know how to recreate my FDM environment if I replace my PC.