Are you talking about the homepage or the search engine?

https://support.mozilla.org/en-US/kb/how-to-set-the-home-page

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, securedsearch

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.

I can not retrieve my bookmarks. I can not even open my e-mail

Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

When done, copy and post the results here.

https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

Hi Ralph, two other questions were posted shortly after yours about a Bing takeover and in at least one of the two cases, bookmark problems (1314107, 1314105). Neither of those threads is solved yet, but it seems that something is going around.