Just go to your settings on your device and find “default browser” and choose one of your liking.

Once you select your default search engine -- this article has the steps:

Change your default search settings in Firefox

-- it should not change on its own, but the setting will be lost if you were to use Firefox's Refresh feature.

Some users have reported search settings being reset when they update if they have used the "Remove" button below the "Search Shortcuts" box on the Settings page to remove ALL of the search engines. Instead of removing all the search engines, you can click in the left column to "un-check" them so they are hidden. (I don't know whether that still happens, actually, it seems pretty rare.)

Finally, add-ons can change your search engine settings. You may want to review/double-check that you don't have any unknown extensions. How? Open the Add-ons page using 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 looks search related, 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.

Any improvement?

I have the search default set as I want it and do not refresh Firefox so as not to loose my settings. It seems random when the ‘evil google’ sneaks in. What a suprise

The next time your default changes, check for any new/unexpected search engines listed in the Search Shortcuts box. There could be a connection with an external program replacing or attempting to replace the search.json.mozlz4 file Firefox uses to store your search configuration.