What is the exact error messages?

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 “ hpdfconverhub “ 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.

Did you look at Tools --> Add-Ons in the Firefox menus? There might be a way to un-install it from there, if it shows up as an extension or plugin. Or perhaps there is an option to disable it. Or, ....

See if it shows up in Windows 10 as an installed program, and use "Uninstall" as you would any program. There's also an "Apps & Features" list under the System Settings that you can uninstall from. I don't use W10 myself, so I don't know if ye goode olde Control Panel Programs and Features method still exists.

This seems counter-intuitive since your pop-up is probably a browser add-on, but sometimes plug-ins are installed as regular Windows software apps instead of installed via the browser's interface.

If none of the above works, then you have 2 other options, both undesirable:

1. Remove and reinstall Firefox from scratch into a clean profile directory. Then re-import your settings from the old profile.

2. Perform plugin brain surgery by deleting the plugin's files revealed by the "about:plugins" page.

Note that it's possible that #2 above could create a gigantic snowball rolling down the mountain with arms and legs sticking out of it, getting bigger and bigger. That's because, if the plugin is just one function of a larger program, there could be other files running on your system related to it, with registry entries as well. If your anti-virus program hiccups over it, then there could be more to it than the annoying pop-up, possibly spyware designed to steal your PDF files. If it's a malicious program, it could have built in protection from being deleted. But that's a little far-fetched, just a remote possibility. (Run a full anti-virus scan as a precaution)

ALSO ... it appears that your Homepage and Search provider list may have been compromised.

Go into "about:preferences" --> Home

  -- make sure that your default homepage settings are correct.

and "about:preferences" --> Search

 -- make sure that your Default Search Engine and One-Click Search Engine settings are correct.

If you correct the settings, and they revert back to malicious settings, then you've got some MALWARE running, either in Windows, or a bad Firefox Add-on. Hopefully your anti-virus program can find and eliminate it.