Hi Carey, I cannot replicate that in a clean install (64-bit Firefox 57 on Windows 10).

As my test page, I used a page from my own site (https://www.userchrome.org/what-is-userchrome-css.html) in the private window, and used one of the links where it says

CSS (download):

That shows up on the download arrow in the private window, and if I click Show All Downloads, it shows up in about:downloads in a new tab.

If I switch over to the regular window, the arrow is not on the toolbar. If I view all downloads (Ctrl+j), it's not listed. Same after closing the private window.

Could you repeat your test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any difference?

Some additional info I neglected to provide: I clicked on the dwhelper plugin icon and it ended up lumped in with all the other downloads. So, maybe a dwhelper issue (?)

Carey said

Some additional info I neglected to provide: I clicked on the dwhelper plugin icon and it ended up lumped in with all the other downloads. So, maybe a dwhelper issue (?)

Maybe. Definitely worth checking their support link on the Add-ons site to see if they are aware of that.