Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; (<Control> [Mac=<Command>] <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Remove “ Bing “ Anything.

Also remove SafeSearch, SearchAssist Incognito, FileConverter Addon, SearchLock, DiscreteSearch, WebSearch, Findwide Search Engine

Now do the same in the computer’s Programs Folder.

Windows: Start > Control Panel > Uninstall Programs. Mac: Open the Applications folder. Linux: Check your user manual.

Mozilla or Firefox does not force Bing to be the search provider at all. Compared to other browsers, it’s even the most customizable one respecting both your wishes and privacy like any other would not. Instead, there is just an issue we should be able to fix, and there could be several causes, as you may have seen when browsing other questions.

First see if an add-on causes the issue by starting Firefox in Safe Mode. If the issue disappears, an add-on is causing the issue.

If that does not help, perform a clean reinstall by removing Firefox using the Add/Remove programs feature in Windows, rename the C:\Program Files\Mozilla Firefox folder to e.g. C:\Program Files\Mozilla Firefox.old, and then run the preferred 64-bit Windows installer in your language that you can download from here.

Other questions have been posted about this issue, such as these:

one (add-on/extension)
two (reinstall)
three (AdAware or Web Companion from Lavasoft)
four (malware)

_{Side note: please be careful when "threatening" to stop using Firefox or any other software in general, especially open-source where users are expected to feed back and help out each other, as the result will often be the opposite of what you expect (people not helping out rather than doing asap hoping not to see you go) and this is merely a sign of people not understanding what an issue is about. ;)}

This could be an issue caused by AdAware or Web Companion from Lavasoft. You can look for a file named dsengine.js in these locations. You should only find channel-prefs.js in the "defaults\pref" location. Any file found here apart from channel-prefs.js is suspicious. You can check the content of the file in a text editor (use open with and do not double-click the file).

C:\Program Files\Mozilla Firefox\defaults\pref\
C:\Program Files (x86)\Mozilla Firefox\defaults\pref\

You can look for a file named dsengine.cfg in the main Firefox program folder.

C:\Program Files\Mozilla Firefox\
C:\Program Files (x86)\Mozilla Firefox\

Delete the dsengine.js and dsengine.cfg files when present.