Resetting Firefox sort out many of problems and keep your firefox into default mode. but it might be loss open pages ,some addons, or other prefences. steps to go : In new tab type about:support . This will redirect you to a new page where in upper right side you get a box that says Reset Firefox . Click here on.

You can do the same with these steps :

   * Go to Firefox > Help > Troubleshooting Information.
   * Click the "Reset Firefox" button.
   * Firefox will close and reset. (After Firefox is done, it will show a window with the information that is imported) 
   * Click Finish.
   * Firefox will open with all factory defaults applied. 

for easy going you can go through this article : Reset firefox

One more thing is there that sometime clearing cookies and cache ,helps you .

* clear out cache : Tools > Options > Advanced > Network > Cached Web Content: "Clear Now
* clear out cookies : Tools > Options > Privacy > Cookies: "Show Cookies"

This issue can be caused by corrupted cookies or cookies that are blocked (check the permissions on the about:permissions page).

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

• Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

• http://kb.mozillazine.org/Cookies
• https://support.mozilla.org/kb/Deleting+cookies

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)