I think I see the button you mean (attached screenshot).

Could you try this:

(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 pdfjs and pause while the list is filtered

(3) If the pdfjs.sidebarViewOnLoad preference is bolded and "modified" or "user set" to to some value other than zero, right-click it (on a one-button mouse Ctrl+click it) and choose Reset. Or you can double-click it and change the value to 0 (that's a zero).

Note: The values mean:

• 0 = Do not display sidebar
• 1 = Show thumbnails
• 2 = Show outline
• 3 = Show attachments

Does that work?

Also, does the change persist after you quit Firefox and start it up again?

Thanks! Unfortunately, Step #3 above is already set to default. I did restart firefox to see if it would help, but the sidebar still appears. Happy to try other ideas.

Maybe toggle pdfjs.showPreviousViewOnLoad to false.

Unfortunately that didn't work either. Thanks, though.

Did you close and restart Firefox after modifying these prefs?

• Mac: Firefox -> Quit Firefox

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox now will open the sidebar under the same circumstances as Adobe Acrobat/Reader (when specified by the PDF itself). For example: Firefox Search Bar Show Engine Names (Firefox 43)_20171004.pdf should open the Bookmarks list.

To suppress that behavior, could you try this:

(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 pdfjs and pause while the list is filtered

(3) Double-click the pdfjs.disablePageMode preference to switch the value from false to true

Now instead of the panel opening automatically, if the PDF specifies that it should be displayed a green dot will appear on the sidebar button. At least, that's what happens in my 15-second test in Firefox 56 using that above-linked PDF.

Let me know what happens for you.

Source: This preference was mentioned (more or less) here: https://github.com/mozilla/pdf.js/issues/8972#issuecomment-333058993

Edit: Fixed link to example PDF.

Jackpot. Thanks, jscher2000!