If you are talking about a plugin, This may be your problem,

Firefox v52+ disables all plugins except for Flash Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work? {web link}

Install Older Version Of Firefox {web link} Be sure to read everything here.

If you still want to downgrade, look under; I still want to downgrade. Click the Directory of other versions and languages link. Look for the directory of the version that you want. But, remember that old versions may have security issues.

You should also check out Firefox; Extended Support Release {web link} ESR Notes System Requirements

AdrianRutter said

I was prompted to download Active 3 exe. today

What is that and who makes it? Can you find that out from the Windows Control Panel "Uninstall a Program", or from Firefox's Add-ons page?

On the Firefox side, you can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious? If in doubt, disable.

Often a link will appear above at least one disabled extension to restart Firefox before the change takes effect. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

Also possibly relevant: Troubleshoot Firefox issues caused by malware.