Please see this KB article: https://support.mozilla.org/en-US/kb/firefox-repeatedly-opens-empty-tabs-or-windows

Thanks. I opened that and under Applications and PDF I noticed Adobe is now gone and only other option shows TWINUI. This happened right after last Firefox Update to 58.02. Before this, whenever I would click on, for example, a bank statement within WellsFargo, it would show Adobe or save file. I had changed this to Open in Firefox which resulted in dozens and dozens of empty tabs loading, downloading. Changed it to this TWINUI, which has pretty much no options. Any ideas of how to solve this sudden absence of Adobe PDF access?

Could you check whether Adobe Reader or Adobe Acrobat is your default PDF program on Windows? Firefox usually will list the default program in the Applications box.

If it won't show up that way, can you click Use other... and select it there? (Screenshot attached showing where that appears.)

I followed your instructions and neither of those Adobes are in the drop down. I selected Use Other, then when Adobe was not listed there I chose Browse. This brought me to Programs. Adobe is not there (except for Adobe Photoshop and Premiere). Makes no sense, right? What do you think I should do? And thank you for helping me.

Did Adobe Reader somehow get uninstalled? You could try the Windows 10 search box and see whether it shows up when you search Adobe. If not, you may need to re-run the installer:

https://get.adobe.com/reader/ Beware of free software offers you may not want in the center column

After re-running the Adobe Reader installer, make sure to open the program and use Help > Check for Updates to get the latest patch.

I think Adobe Reader was somehow uninstalled. What is the difference between Adobe Flash Player and Adobe Reader? I'm only wanting to use Adobe for viewing and printing PDFs. Once I get your answer on the difference, I'll try to install from your link, making sure not to allow any free software and also checking for updates. Thanks!

Those are completely different Adobe products.

Adobe Flash Player -- which on Firefox is called the "Shockwave Flash" plugin -- is used by some sites for embedded videos and games. On some older sites, the entire site runs inside the Flash plugin (years ago this was common for fancy hotel and restaurant sites). Flash is never used for PDFs.

I have installed Adobe Acrobat Reader DC. It's asking if I want it to make it my default PDF application. Should I do that now or wait to see if it shows up in the Firefox Application PDF drop down box? I don't want to mess up my computer like I did by selecting Application/PDF and View in Firefox.

gramto4 said

I have installed Adobe Acrobat Reader DC. It's asking if I want it to make it my default PDF application. Should I do that now or wait to see if it shows up in the Firefox Application PDF drop down box?

You may as well assign it now. Unless there is a different PDF viewer that you prefer to open PDFs that you've saved to disk.

Okay, Adobe Acrobat Reader DC shows up in Other. Not sure what to do now.

If you select it in other, does it seem to stick?

Recently, the only way I've been able to view PDFs is with that TWINUI, which is not something I like. Before this, Firefox updated to 58.0.2 and Windows did it's latest updates. And then poof! No more viewing PDFs in Adobe.

I clicked on Adobe Acrobat Reader DC in OTHER, closed options, reopened and looked again under Firefox Applications/PDF and it is listed now as default. Is that what you mean by sticking? Not sure what you meant by "a different PDF viewer that you prefer to open PDFs that you've saved to disk". Would the TWINUI that Firefox Application showed be that? I didn't save that to anything, not even sure where it came from.

Yes, that was the idea, you may prefer PDFs to open in Adobe Reader when launched from your Downloads folder or other locations on disk.

TWINUI? I see it isn't always welcome: https://www.google.com/search?q=TWINUI+site:answers.microsoft.com

I want to thank you for all your help. You solved my problem. I had no idea what I was doing (fairly obvious) but that did not stop you from sending info. It appears from the link you sent on TWINUI that it isn't very likeable. No way of knowing if the Firefox update caused Adobe to be replaced with TWINUI or Windows Update. Lesson to learn for me: write down the process of getting it back, thanks to you.