I think you should be able to change the size of the window. Try this:

1. Open a new window with facebook logged in
2. Resize the window
3. Close the window
4. Restart Firefox
5. Try the share button to Facebook

Did that change the size of the window?

Resizing the window is exactly what I cannot do !!!

The problem is not solved and I see no field to say so.

I cannot resize a pop up window in firefox. It jumps back to its big size.

Changing the size of the window is the problem.

Got it, my apologies for not getting that part. You can delete this folder in your profile to get rid of any corrupt default window sizes: Delete xulstore.json file

• https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved#firefox:win10:fx57

Restart Firefox after deleting the file, if the file is not there, try deleting all the stored cookies for the site. It will be under General of the Control Center (accessed from the taskbar) Site Information panel

If you do not mind sharing the url and the link where this happens, we can also confirm if this is just a feature of the website.

Thank you!

Can you open the system menu via Alt+Space?

Try to rename/remove xulstore.json in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox
• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\

"AppData" (XP: Application Data; Mac: ~/Library; Linux: ~/.mozilla) is a hidden folder.

• http://kb.mozillazine.org/Show_hidden_files_and_folders

I deleted that file; it was easy to find (( xulstore.json )) But it did not enable me to resize the facebook window from either msnbc.com or nbc.com. However, I did open the window to full size and the share button was accessible. Now, I do not see how to empty cookies in firefox .

You can remove the cookies via the Options page.

• Options/Preferences -> Privacy & Security -> "Use custom settings for history" -> Cookies: "Show Cookies"

You can inspect cookies and passwords and permissions for the domain in the currently selected tab via these steps:

• click the Control Center 'i' icon at the left end of the location/address bar
• click the arrow to expand the security message
• click "More Information" to open "Tools -> Page Info"
• click "Security" to inspect and modify cookies and passwords and inspect the certificate chain in a standalone window
• click "Permissions" to inspect and modify permissions

• click "View Cookies" in "Tools -> Page Info -> Security" to inspect the cookies for the domain in the currently selected tab

Clear the search bar to see all cookies.