Basic instructions for changing your home page (and start page, which aren't necessarily the same thing):
With just a single tab open, navigate to the page you want to use.
Go to Tools > Options and (if necessary) select the 'General' tab.
Under 'Home page' select 'Use current pages'.
Next to 'When Firefox starts' choose 'Show my home page'.
Click 'OK'
If that doesn't work you've got malware (in the form of a browser hijacker) on your computer. Download, install and run the free version of Malwarebytes Anti-Malware, which will hopefully get rid of it:
https://www.malwarebytes.org/antimalware/
(NB: When installing it, take care to deselect the pre-ticked option to take a free trial of the Premium version. If malware is detected ensure that you quarantine everything found).
After running Malwarebytes, use the instructions in my first paragraph again.