Try reinstalling Firefox with the latest installer so that it registers as a PDF viewer.

If for the purposes of an attachment I click on "Open with", Firefox does not appear among the viewers offered.

Two questions:

(1) Could you describe in more detail where you are having problems using Open With on an attachment? For example:

• In a download dialog in Firefox (Open/Save/Cancel dialog)
• On the Gmail site, Outlook site, Yahoo site, using a download/view feature in the web page
• In the stand-alone Mozilla Thunderbird application, Microsoft Office Outlook application, or other mail program
• In a folder you're viewing in the Windows 10 File Explorer program
• Somewhere else

(2) Is the built-in viewer ("Open in Firefox") available in the download settings? See: View PDF files using Firefox’s built-in viewer.

This is a different selection than the "Use other" feature set to firefox.exe, which can cause an infinite loop. See: Firefox repeatedly opens empty tabs or windows after you click on a link.

Thank you, but to re-install must I first delete Firefox as installed currently?

You don't need to uninstall first, just run the installer.

My attempt to reinstall was thwarted by the message that I already have the latest version. Therefore I still cannot access the Firefox PDF viewer. Might it be worth uninstalling my current Firefox and then installing anew?

In answer to jscher2000, for whose message I'm grateful, it's on the Gmail site that "Open in Firefox" is not being offered as a PDF viewer option and, when I try to download a PDF, by clicking on the usual downward-pointing arrow, nothing at all happens.

... it's on the Gmail site that "Open in Firefox" is not being offered as a PDF viewer option and, when I try to download a PDF, by clicking on the usual downward-pointing arrow, nothing at all happens.

Thank you for the added detail. As an example:

When you initially click the attachment, Gmail usually generate its own preview/overlay rather than sending the PDF to Firefox's viewer. The Gmail viewer has a download arrow on the upper right --

-- but I suppose you have the same issue with that button as the other one.

If you check the Downloads arrow on the main toolbar, is Firefox auto-saving the attachments to disk without opening them?

If Firefox isn't receiving the file, one possible reason is that the request isn't going through. Google uses a different server to store attachments, and uses a redirect to get the file. Generally speaking, that looks like this:

clicking the download arrow sends a request to:

https://mail.google.com/mail/u/0/?parameters

that server sends a 302 Temporary Redirect to

https://mail-attachment.googleusercontent.com/attachment/u/0/?parameters

Normally, Firefox happily follows that redirect, but if you have a custom setting or add-on that blocks redirects, or if you have privacy add-ons that strip off any of the parameters on the long URLs, then the request may fail.

We typically suggest testing in Firefox's Troubleshoot Mode to bypass the effects of add-ons. More info in this article (please note, don't use the Refresh/Reset button unless/until you have given up on diagnosis):

Diagnose Firefox issues using Troubleshoot Mode

Many thanks.

However, I'm afraid that I'm getting beyond my technical comfort zone and so I propose to defer any further steps* pending the visit at the weekend of one of my sons, who is much more tech savvy than I am.

• Unless it might be safe and sensible for me to uninstall my current Firefox and then reinstall Firefox with the latest installer.

Since the problem only affects one site, it seems a little drastic to zero out your Firefox and start from scratch. Perhaps before the weekend, you could do another test, which is non-destructive. This involves creating a test profile. Here's how:

(1) Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

(2) Click the "Create a New Profile" button, then click Next. Assign a name like Test2024, ignore the option to relocate the profile folder, and click the Finish button.

(3) Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

(4) Scroll down to Test2024 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

(5) Sign in to Gmail and see whether the PDF viewer works normally in the test profile.

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2024 will remain available for future testing.)