(To other readers finding this thread, please start with the support article: View PDF files using Firefox’s built-in viewer)

Hi dawger1, does this affect all PDFs from all sites?

Some sites may not send the specific content type header ("application/pdf") that triggers the built-in viewer, so it's helpful to see whether it's that issue before diving into extended troubleshooting.

Application Settings

You definitely should have Portable Document Format (PDF) in the box. It's one of the basics that Firefox creates at installation or in a new profile:

If that does not appear, it's possible the settings file which stores your download preferences has become corrupted. You can rename the file and have Firefox rebuild it. Here's how:

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

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

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

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

• "3-bar" menu button > Exit
• (menu bar) File > Exit

Pause while Firefox finishes its cleanup, then right-click > Rename handlers.json to something like handlersOLD.json. If you see a file named mimeTypes.rdf, rename that one to mimeTypesOLD.rdf.

Start Firefox back up again. Any improvement?

Is pdfjs disabled?

While it is unlikely, you could check the setting here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste pdf and pause while the list is filtered

(3) If the pdfjs.disabled preference is bolded and "modified" or "user set" to true, double-click it to restore the default value of false

Any progress so far?

Hi jscher2000,

Thank you very much for the quick response! I'm unable to view PDFs from any website within Firefox.

I should've added that I removed and re-installed Firefox yesterday afternoon in an effort to solve this issue, to no avail.

I renamed handlers.json in an attempt to rebuild the file as you suggested, but the PDF/Portable Document Format content type was still not listed after restarting Firefox.

I checked pdfjs.disabled, and it was set to true initially. However, changing it to false made no difference in the browsing session. Clicking on links/opening bookmarks with PDFs results in an Open With/Save File dialog box. In fact, when I closed Firefox and re-opened it again, pdfjs.disabled was automatically re-set to true.

So at a high level, I see two main issues: 1. The Portable Document Format not being listed as a content type; and 2. The built in PDF viewer is automatically resetting to disabled each time I open the browser

dawger1 said

I checked pdfjs.disabled, and it was set to true initially. However, changing it to false made no difference in the browsing session. Clicking on links/opening bookmarks with PDFs results in an Open With/Save File dialog box. In fact, when I closed Firefox and re-opened it again, pdfjs.disabled was automatically re-set to true.

Are you using a "managed" version of Firefox that could have its features modified by a company IT department?

If not, there are a number of reasons that a settings change you make during your session might be lost between sessions. This article discusses a number of potential explanations/solutions: How to fix preferences that won't save.

I'm not sure what happened, but Portable Document Format is now listed as a Content Type in my copy of Firefox, and I'm able to preview within the web browser. I followed the instructions to rebuild the handlers.json file earlier today, but that's all I've done to the laptop.

Thank you for your help, jscher2000!