Hi  ! You may be dealing with search.yahoo.com, which is a browser hijacker, malware, that you of course wil want to get rid of.

Go to the 3-bar menu (or   'Tools') => Addons => Extensions and look for any unfamiliar or suspicious looking extensions, like e.g. :

SafeSearch Incognito SearchAssist Incognito Search for Fire Fox 1.6 SafeSearch ch SafeSearch net File Convertor and Search Addon

When you find any : remove them.

Do the same in Windows' Control Panel => Programs.

However :   most malware will be 'hiding' somewhere , so let's run malware scans :

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 malware scanners.
They all work differently   -   what one program doesn't pick up, the other might.

Also see : https://malwaretips.com/blogs/yahoo-toolbar-removal/

Thanks for guiding me but I got another problem in middle of the process!!! Now instead of another undesired search engine I get "The address isn’t valid" or "Server not found"!

what did i done wrong?

my problem differs from a simple redirection! It seems that my problem has nothing to do with my DNS because search engine changes in options section when I close or lauch FF. I mean I launch FF and delete that undesired search engine, then close FF, then after a few minutes relaunch FF and go to options section and notice that undesired search engine is present there!

Also as you see in this pictures, "Yahoo" and "Default Search" are two separate search engines which confused me!

The other picture lists all of my addons, any of them lookslike a malware?

esmetozerbezan said

Thanks for guiding me but I got another problem in middle of the process!!! Now instead of another undesired search engine I get "The address isn’t valid" or "Server not found"!

Today's not your lucky day, is it  ?

See : https://support.mozilla.org/en-US/kb/server-not-found-connection-problem

Also as you see in this pictures, "Yahoo" and "Default Search" are two separate search engines which confused me!

Remove 'Default Search' and 'Yahoo' (legitimate search engine that you don't want).

The other picture lists all of my addons, any of them lookslike a malware?

Remove every extension that you don't recognize, don't use, didn't install yourself.

Look, now firefox loads websites fine but when I type something in url bar, it adds "www" and ".com" to it and tries to navigate there! Somewhere in this website I read about changing a value from "true" to "false" in "about:config". I think that messed my situation, but unfortunately I don't remember what value I changed!

I removed extensions that I don't use and also removed all search engines except Google, then restarted FF and now there is no trace of the problem. Now It seems that I just have above problem.

esmetozerbezan said

Look, now firefox loads websites fine but when I type something in url bar, it adds "www" and ".com" to it and tries to navigate there! Somewhere in this website I read about changing a value from "true" to "false" in "about:config". I think that messed my situation, but unfortunately I don't remember what value I changed! I removed extensions that I don't use and also removed all search engines except Google, then restarted FF and now there is no trace of the problem. Now It seems that I just have above problem.

Well, the problem you started this thread about is solved - Yay  !

As for the other problem : would you be so kind as to start a new thread, as this is a different topic - you can do so by clicking 'Ask a question' at the top of this page.

Happy112 said

esmetozerbezan said

Look, now firefox loads websites fine but when I type something in url bar, it adds "www" and ".com" to it and tries to navigate there! Somewhere in this website I read about changing a value from "true" to "false" in "about:config". I think that messed my situation, but unfortunately I don't remember what value I changed! I removed extensions that I don't use and also removed all search engines except Google, then restarted FF and now there is no trace of the problem. Now It seems that I just have above problem.

Well, the problem you started this thread about is solved - Yay  !

As for the other problem : would you be so kind as to start a new thread, as this is a different topic - you can do so by clicking 'Ask a question' at the top of this page.

Well, it is not known that my problem is gone! Previously I had removed all search engines except Google but that search engine had returned back. Possibly the problem is in there and after returning that value returns back but ok, I am going to ask it separately.

Quoting you :

I removed extensions that I don't use and also removed all search engines except Google, then restarted FF and now there is no trace of the problem. Now It seems that I just have above problem."

When you say that now there is no trace of the problem , I assume that means : problem solved ..........

Happy112 said

Quoting you : I removed extensions that I don't use and also removed all search engines except Google, then restarted FF and now there is no trace of the problem. Now It seems that I just have above problem." When you say that now there is no trace of the problem , I assume that means : problem solved ..........

Previously I was getting redirected to some undesired search engine, now I am getting redirected to an empty space which is worse! This means that still my problem with search bar exist! Unfortunately my problem with search engine just has converted from one sort to another one!

I mean maybe when FF's search bar tries to function, instead of using my chosen search engine, adds and uses another one. Now search bar does not even function to see whether problem is gone or not!

Try to remember when this problem started , then go to windows system protection , and choose to restore your system to a previous date

thagoodboy said

Try to remember when this problem started , then go to windows system protection , and choose to restore your system to a previous date

System restore has been disabled long ago! Because It takes a lot of space that I don't have to waste! Anyway thanks for trying to help me.

Plus, as I wrote it before, anyway I went and asked about the solution and got the answer here: https://support.mozilla.org/en-US/questions/1180375

Now the problem is gone, so my special thanks goes to Happy112 for solving my main problem!

This might be helpful to others: I had removed adware protection and proxy extensions that are present in one of those pictures above.

@ esmetozerbezan :

You've been through a lot these past few days, and I'm so glad to see that both problems have been solved  !