Yes, this is an unfortunate side effect of the general change to the downloads workflow in Firefox. After Firefox saves the file, you can click the Downloads button on the main toolbar and open the PDF from there.

So I can't make it work like before anymore?

There's a catch-22 with PDFs: When you set the behavior to "Open in Firefox" to preview PDFs, then you can't get the dialog for the viewer's download button. If you change the behavior to "Always Ask", then you can't get the preview seamlessly as web content when opening a PDF. I don't know whether anyone has come up with a workaround for this. You could experiment with Always Ask and see what you think.

Forgot to link this article: View PDF files using Firefox’s built-in viewer

On both computers (updated firefox and not-updated firefox) I have both settings in firefox preferences set for PDF open in firefox. With this setting when I click in toolbar download button it was always set to "always ask" so it asked me whether I want to open file with Acrobat or whether I want to save file. Now it saves file. How do I reverse it back? What changed with this update?

This update significantly changed the way downloading works: How file downloads are handled in Firefox. I'm not sure why it had to change the download button in the PDF viewer, but I don't know of a way to reconcile the conflict.

During rollout and debugging, there is a temporary preference to revert many/most of the changes (about:config => browser.download.improvements_to_download_panel), but I haven't tested how it works with the PDF viewer.

Note that it isn't recommended to disable a feature by toggling a master control pref like in this case browser.download.improvements_to_download_panel as it isn't guaranteed that this will keep working properly in the future when old code is being removed and the new code is the only one that is available.

This update significantly changed the way downloading works: How file downloads are handled in Firefox. I'm not sure why it had to change the download button in the PDF viewer, but I don't know of a way to reconcile the conflict. During rollout and debugging, there is a temporary preference to revert many/most of the changes (about:config => browser.download.improvements_to_download_panel), but I haven't tested how it works with the PDF viewer.

Note that it isn't recommended to disable a feature by toggling a master control pref like in this case browser.download.improvements_to_download_panel as it isn't guaranteed that this will keep working properly in the future when old code is being removed and the new code is the only one that is available.

yes!!! it worked!!! Thank you so much :)

I ended up disabling the global option as well, and it worked:

browser.download.improvements_to_download_panel == false

This issue MUST be fixed. I frequently review papers for conferences, and I need the ability to pop up Acrobat on demand -- without always popping up Acrobat. This used to work, but now is hopelessly broken (i.e. If I hit the download button from the Firefox viewer, it forces me to pick a place to download, then downloads; when I click on the downloaded file, it opens with Firefox again! So, only way to make this sorta work seems to be to download, then "open containing folder", then double-click on the file in the file browser. ACK!).

Please fix. For now, I've globally disabled improvements, which will doubtlessly break in the future...

This is not an enhancement, it's broken my workflow.

I don't always need to download a pdf, sometimes I just need to check it in preview.

I download files to Adobe to fill, sign and then save to a folder. When I load them back to the server, I used Firefox preview to make sure that the signing was correct. Now I have to download and reopen the file AGAIN in Adobe to check it. My folder is now cluttered up with the pdfs that aren't signed and saved.

PDF handling is now cumbersome and kludgy, not optimized at all.