Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

I tried everything but still it didn't work. Can you please help me how to unfill the find bar with selected text when opening it  ?

The find bar is mean to be filled with the current selection when opened (Bug 250910). You can clear it by pressing the Backspace or Delete key.

It doesn't work but whenever I reopen the find bar the highlighted word or letter shows. Can you please help me what to do ?

Can you select / highlight the word or letter previously searched?

But I wanted blankly opened the find bar

Firefox remembers what was last used in the find bar in other tabs and shows this as selected text to allow you repeat this previous search and overwrite it if you want a new search. The only way to get an empty find bar is by typing something and then delete it with backspace and close the find bar.

Still it does not solve my problem. Still the highlighted word or letter comes. Can you please help me what to do ?

You can't prevent getting highlighted/selected text if you open the Find Bar and you previously used the FB in another tab and didn't clear its content before closing the FB, that is how the FB works like I wrote above.

This is what I find whenever I reopen the find bar

Maybe disable the Quick Find bar (FAYT) feature.

• Settings -> General -> Browsing -> Search for text when you start typing

You can look at these prefs in about:config to disable FAYT for cases like ' and / to open Quick Find for links.

• accessibility.typeaheadfind => false
• accessibility.typeaheadfind.manual => false