Type about:preferences#search<enter> in the address bar. You can select any search engine to be your default from here.

You can also remove any engine from here.

If you want to add a search engine, check this link: https://mycroftproject.com/

Hi, to add Startpage, you can open their home page at https://www.startpage.com/, then right-click the address bar, and add it from the item at the bottom of the context menu.

For Yahoo, most likely it was added by 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 that mentions search, or privacy, or looks 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 no add-ons are listed?

Sometimes you need to restart in Firefox's Safe/Troubleshoot Mode -- that temporarily deactivates extensions -- to fully use the add-ons page. To do that, use 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).

Can you flush it?