I tried those links from a W10 computer and have no problem navigating them. I have FF57.0.4x64 so not sure what your seeing unless you can provide a screen shot of the error or step by step so it can be reproduced by other Linux computer to verify the problem. But if they don't have this issue then it would be a local computer issue not a FF issue.

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

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache:

• Options/Preferences -> Privacy & Security -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Options/Preferences -> Privacy & Security -> "Use custom settings for history" -> Cookies: "Show Cookies"

Thanks for the feedback. @cor-el: When I said "with add-ons disabled" I was doing that already via Safe Mode, also tried with a fresh profile (to avoid losing cache+cookies), no difference.

Found out a bit more:

• On manufactum.com I can open the links in a new tab using Ctrl, not sure why that didn't work before. But middle-click still won't work.
• On userstyles.org Ctrl-click the main navigation opens the links the same window, middle-click shows the pop-up blocked icon in the address bar.

Still have to test on a Windows machine.

So you are saying that on both those websites you can open the links of the main navigation in new tabs with both Ctrl and middle-click? Weird…

Thanks!