With Firefox Nightly 96.0a1 (2021-11-06) (64-bit), saving a PDF from a response with Content-Disposition: attachment results in a 0-byte file with the chosen filename and a PDF file with -1 appended to the name which is also opened in a new tab. Steps to reproduce:

1. Set browser.download.useDownloadDir:false in about:config (or by selecting "Always ask where to save files" in about:preferences)
2. Visit https://developer.arm.com/documentation/aeg0014/g
3. Click the download button.
4. Click the "ARM Glossary" link in the Downloads modal window which appeared.
5. Press "Save".

Previous versions of Firefox would save AEG0014G_ARM_corporate_glossary.pdf as requested. Current versions create a 0-byte AEG0014G_ARM_corporate_glossary.pdf and save the content in AEG0014G_ARM_corporate_glossary-1.pdf then display the saved content in a new tab. Mozregression:

Last good revision: 216de3cdebc9db25336b56380f23a20252d7e555
First bad revision: 3de56e38f3c87f33a1e7849701edb3c62bc472a5
Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=216de3cdebc9db25336b56380f23a20252d7e555&tochange=3de56e38f3c87f33a1e7849701edb3c62bc472a5

Which suggests this was regressed by Bug 1732347. I can confirm that with browser.download.improvements_to_download_panel:false the issue does not occur. Mozregression with browser.download.improvements_to_download_panel:true:

Last good revision: 141832e40a4c643e9a68c21e5d109b42baed7736
First bad revision: 16afe3f983133f2df1242302cff2a5c0e35c3838
Pushlog: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=141832e40a4c643e9a68c21e5d109b42baed7736&tochange=16afe3f983133f2df1242302cff2a5c0e35c3838

Perhaps the issue was introduced by Bug 1719901?