I need help in a couple of incidents. I have lost my usual browser, Firefox to Searchdconvertnow.com and do not know how to remove it.

I use Ubuntu 16.4 ? But I ma also 78 and not up to speed on computers.

The second problem is that I have changed my mail to Telstra ID and the password and name is set and functioning; however when I try to up the page it does not work, so I cannot see all the emails.

The page up and down is not working on some other sites as well. Maybe if I get rid of the search convert program all will be well again.

That's a notorious piece of malware and a warning not to download odd things off the web! The quickest thing to do is
1. use your package manager to uninstall Firefox.
2. delete the folder .mozilla — you can find that in your file manager by enabling the "show hidden files" option. That will remove the various bits of the plugin from whereever they're hidden.
3. go back to the package manager and re-install Firefox. That will also get you a new .mozilla folder created when it sees that there isn't one.

Without knowing your distro, it is kind of hard to give specific instructions, but DavidMcCann advice is solid. Assuming you are using Ubuntu as I see it it mentioned below your name, and are up to date, to get this done from a terminal quickly do:
and you should be good to go

A good example of the fact that with Linux you can get things done in many ways. The last poster likes the command line, I prefer the GUI, but we both get there in the end!

If you still have problems with emails, then post again with a new thread specifically about that to catch the eye of more people.

PS I'm in the 70s too — you're never too old!