Sounds like a corrupted FF Browser. Did this happen before?

A few weeks now.

I have to get my grandson. Sorry. Try again later.

One possible culprit would be an extension. 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.

Then cast a critical eye over the list on the right side. 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).

Once interfering add-ons are subdued:

This article has steps for updating your home page setting if needed: How to set the home page

Any improvement?

Oh, also, you might need to update your PDF settings in Firefox here: View PDF files using Firefox’s built-in viewer.