I have same problem when I try to view Facebook pages which are public. No trouble viewing them in past. Tried opening FF with add- ons disabled, still showed overlay on screen. This is text on overlay screen. (I removed name of Facebook site)

"See more of************* by logging into Facebook Message this Page, learn about upcoming events and more. If you don't have a Facebook account, you can create one to see more of this Page. Sign Up Log In"

This code in userContent.css seems to work at the moment to hide this message.

Add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

@-moz-document domain(www.facebook.com){div[class="_5hn6"]{display:none!important;}}

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

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

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

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

I am going to consider this as RESOLVED although there appears to be NO FIX to me problem. I guess I will just have to contend with the SIGN UP/LOG IN popup window until either something changes or, just maybe, switch to a different browser. I don't use Chrome. So I don't how Chrome has anything to do with using Firefox (on Linux).

The chrome name of that folder that contains userContent.css has nothing to do with Google Chrome. Firefox uses the "chrome://" protocol to access its own internal files and also access files from extensions (no relation with Google Chrome).

• https://developer.mozilla.org/en/Chrome

You can use the Stylish extension to make it easier to manage custom style rules if you do not want to use userContent.css.

• Stylish: https://addons.mozilla.org/firefox/addon/stylish/
• http://userstyles.org/help/stylish_firefox

You can use these steps to create the chrome folder and the userContent.css file in it.

• create the chrome folder (lowercase) in the <xxxxxxxx>.default profile folder if this folder doesn't exist
• use a plain text editor like Notepad to create a (new) userContent.css file in the chrome folder (file name is case sensitive)
• paste the code in the userContent.css file in the editor window
• make sure that you select "All files" and not "Text files" when you save the file via "Save file as" in the text editor as userContent.css.
  otherwise Windows may add a hidden .txt file extension and you end up with a not working userContent.css.txt file

I tried again by following your steps. This time it appears that this fix does work. I am wondering if this problem with Facebook is in Firefox or with Facebook. No matter I guess. I seem to have ... for now ... fixed my problem. Also, I just saw that Mozilla has released Firefox v47.x. I will have to wait for the Linux repository/ies to test and release v47 for updating Linux distros. Maybe this problem is addressed/fixed in v47.

I was able to stop the Facebook overlay by adding a custom filter to the Ad Block Plus FF add on. The filter that I am using is:

facebook.com###pagelet_growth_expanding_cta

I needed to create new backup and save after I added filter so that new filter would not be deleted every time FF closed. There are other filters to try on Ad Block Plus forums.

It is Facebook that is doing this and you should see this in all browser unless you have a content blocking extension that catches this.

For PC users

Replace www.facebook.com/...... by m.facebook.com/....... than you will get the mobile version in your Firefox browser.