Make sure you not running Firefox in (permanent) Private Browsing mode.

• https://support.mozilla.org/kb/Private+Browsing
• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use private browsing mode"

If you not using Private Browsing Mode

• Tools > Options > Privacy > History>
• Firefox will: "Use custom settings for history":
• Keep until "They Expire"
• Deselect: [ ] "Clear history when Firefox closes"
• https://support.mozilla.org/kb/Clear+Recent+History

If you use cleanup software like CCleaner then check the settings for the Firefox application.

________________________________________________________________________________

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.
• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks for reply, I already checked and performed what you have mention, but no success

Create a new profile as a test to check if your current profile is causing the problems. See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

Profile Backup and Restore

• http://kb.mozillazine.org/Profile_backup
• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Okay, I will do that, sould I also revert the changes which I did in about:config ?

I don't know is this right time to say this or not but looks like Creating-a-new-profile thing is working for me, and thanks for it. The Cached Web Content was present even when I restarted browser, and even when I restarted PC, this was nothing suprise as same was happening earlier too but when I agains shutdown my PC and after 2-3mins restarted it, the Cached Web Content was still present there.

I can only insure that this was the real Fix of the problem I posted after few hours, if it still works, I will post it here and select the Creating-a-new-profile thing as the best answer.

All I need to do now is to shift my bookmarks from old profile to new one. Thanks for the help.

Yes, the solution of creating new profile is working!