Just checked. I have Win 11 Home and Firefox 150 and all is fine.

pdfs are set Always to be opened by Firefox. i.e. Right click any pdf > Open with > Choose another app > Firefox [Always]

I went to a folder, selected a pdf and it opened in Firefox, though Firefox is open as I am answering this :)

Thanks for this. I've tried setting pdfs to be 'Always opened by Firefox' but it doesn't solve the problem.

I normally have locally stored pdfs set to Always open with Acrobat, but sometimes use the right click to choose Firefox instead, and this now gives the blank pages

For downloads from the internet I have Firefox set to 'Always Ask'. When I select Open with Firefox it now gives blank pages. If I select Download it downloads correctly and is readable in Acrobat, but is blank if then opened in Firefox. I have tried changing the pdf setting in Firefox to 'Open in Firefox' rather than 'Always ask', but the problem persists.

"I have tried changing the pdf setting in Firefox to 'Open in Firefox' rather than 'Always ask', but the problem persists."

That's just for when you are already using Firefox to download.

The fact that the page is blank seems as though Firefox doesn't have the correct interpretor to read the content.

Have you completely reset the computer, i.e. [Restart] if Windows, not [Shut down] Shut down keeps old settings.

Many thanks Amoun. I've done a Restart and it's all OK now.