Sounds like you're infected and need to scan. To get an idea of what's running on the computer, I would use these tools to see what apps are set to run(autoruns) when the computer starts. Then use process explorer to see the processes and verify what corp they are from. When you open Firefox, there should be a process that stands out. No install required, just download and run.

https://docs.microsoft.com/en-us/sysinternals/downloads/autoruns https://docs.microsoft.com/en-us/sysinternals/downloads/process-explorer

The first page shown at startup can be:

• Home page (see: How to set the home page
• Pinned tab page retained from a previous session (see: Pinned Tabs - keep favorite websites open and just a click away
• Page restored from previous session (either due to crash recovery or having Firefox set to restore your previous session)
• Page loaded by an add-on*
• URL specified in the desktop shortcut you use to start Firefox
  in the Target line: "C:\Program Files\Mozilla Firefox\firefox.exe" https://www.example.com/
• Possibly loaded by a scheduled task (Windows task scheduler)

*If you haven't recently, here's how to check your add-ons. Call up the Add-ons page:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• Command+Shift+a (Windows: Ctrl+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.

What if nothing is listed?! This can occur if an extension willfully interferes with display of the page. To disable interference, restart Firefox in its Safe/Troubleshoot Mode using 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: The "Refresh" option is the last resort. Please review the following article to understand what will be deleted: Refresh Firefox - reset add-ons and settings.

Any improvement?