I got Acrobat and plugin and sites I visit with pdf files can be open without problem. So what exactly is the error that it says? And is your Acrobat the latest version? Why do you need that site to look for Acrobat plugin that should be installed from Acrobat when you installed it. You should best check Adobe Acrobat site itself for updates to Acrobat that will update your plugin at the same time.

mark81957 said

Question - is there any way to get an Adobe plug-in for FF58 or, if necessary - can I get a older version of FF that supports adobe plug-in?

Firefox 58, no.

Firefox 52 Extended Support Release, yes.

However, it may be easier to open the PDF outside of Firefox in Adobe Reader/Adobe Acrobat rather than viewing it in a tab. Will they let you do that, or do they make it hard to download?

WestEnd said

I got Acrobat and plugin and sites I visit with pdf files can be open without problem. So what exactly is the error that it says? And is your Acrobat the latest version? Why do you need that site to look for Acrobat plugin that should be installed from Acrobat when you installed it. You should best check Adobe Acrobat site itself for updates to Acrobat that will update your plugin at the same time.

Please read https://support.mozilla.org/en-US/kb/npapi-plugins so you can better understand about Plugins working in Firefox.

Short version, Firefox Releases since 52.0 which includes 53.0, 54.0, 55.0, 56.0, 57.0, 58.0 and later only allows the Flash Player Plugin from Adobe to run. Only the Firefox 52 ESR allows other Plugins if you use 32-bit Firefox 52.0 ESR on Windows as Win64 only allows Flash Player and Silverlight to run.