What is the content of that pinned tab?

• Firefox View, a recently added built-in page
• A page you pinned once, but then closed or unpinned
• A page you never pinned yourself that something keeps re-pinning

If it's that third category, one possible culprit would be an add-on. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

If the extensions list is blank or blocked:

Restart Firefox in its Troubleshoot Mode to bypass interference by add-ons. Either:

• "3-bar" menu button > Help > Troubleshoot Mode... (before Fx88: Restart with Add-ons Disabled)
• (menu bar) Help menu > Troubleshoot Mode... (before Fx88: Restart with Add-ons Disabled)

and OK the restart. A small dialog should appear. Click the Open button (before Fx88: "Start in Safe Mode" button).

Note: Don't use the Refresh without first reviewing this article to understand what will be deleted: Refresh Firefox - reset add-ons and settings.

Can you disable/remove all the possibly untrustworthy add-ons and, if so, any improvement at the next normal startup?

By the way, the other thread seems to be about the Shortcuts section of the built-in Firefox Home page which appears on each new tab by default. If the problem is that you want to remove a shortcut, check out this article instead: Customize items on your Firefox New Tab page.