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

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.

If you think any website's you use have been compromised:

1) Change your password 2) Confirm your e-mail and other data on that site are correct