Correction: I made an error in the text above, it should be:

file:///C:/folder/a.pdf#page=3

but the problem prevails.. Sorry!!

Could you check whether Firefox is set to open PDFs in a tab. This article leads the way:

View PDF files using Firefox’s built-in viewer

Hopefully that will restore the previous behavior but, if not, it's possible that the file which stores application handler preferences has become corrupted. The file has a somewhat inscrutable XML format so it's often easiest simply to hide it from Firefox and have those settings return to their defaults. Here's how you do that:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, click the "Show Folder" / "Show in Finder" / "Open Directory" button. This should launch a new window listing various files and folders.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit (or Firefox > Quit)

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to mimeTypes.old.

Start Firefox back up again, and Firefox should rebuild the file with default settings. You probably need to switch your PDF handler on the Options page from the internal PDF viewer to the Adobe Acrobat plugin. See: View PDF files using Firefox’s built-in viewer.

Then... success?

The rebuild of mimiTypes.rdf resolved the problem. Acrobat* disappeared from applications. Maybe this was the problem: https://helpx.adobe.com/fi/acrobat/kb/link-html-pdf-page-acrobat.html It is in Finnish and says a URL C:/folder/a.pdf#Page=3 does not work with the page number. Thank You!