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.

what can i do next to avoid the loss of personal data as there has been a ransomware attack as well. How can one avoid malware attacks on firefox as well , i had pretty good anti malware software which detected it but still someone was able to steal the info.

There are system-level keyloggers, of course, that's the classic.

One way to log keystrokes in a browser specifically would be using an extension. Extensions do not have direct access to Saved Logins, but they can inject scripts into web pages. You might have one or more extensions installed that have the highest permission of "Access your data for all websites". While the vast majority of extensions respect you and your privacy, some malware extensions may watch your form entries and exfiltrate what you entered to remote servers. Keep a close eye on your extension collection and be cautious about adding new ones.

• Tips for assessing the safety of an extension
• Other advice on badly behaved extensions: Remove a toolbar that has taken over your Firefox search or home page
• Detailed guidance from Google for IT people: Understand the risks of permissions for Chrome extensions

i had an anti malware/adware browser extension with total security , yet someone at able to steal info about users/passwords , and add keylogger using some add pop ups , is that possible with firefox ?

how can i ensure that the malware is still not existing in the system or browser ? and Secondly how can i know about the damages with the stolen info and mitigate it ?

Have you tried the malware scanners I wrote about?

As to the stolen data, all we can recommend is you change ALL passwords immediately and verify your personal data on those websites.

Thanks Fred.

Just download each scanner you want to use and run it. All are free to use. A subscription opens special features and options.