WORKED!!! Rated 5 out of 5 stars

At first didn't work but tried kzk0813 instructions and worked:

Fix Gmail Manager Can't connect Google ServiceLoginAuth

1) Save this text to a notepad empty file then Close Firefox
2) go to your profile C:\Users\[your account]\AppData\Roaming\Mozilla\Firefox\Profiles\[some random text].default\extensions
3) {582195F5-92E7-40a0-A127-DB71295901D7}.xpi rename with ZIP extension
4) inside this archive under folder named component edit this file named gmServiceGmail.js with notepad
5) Search inside this file gmServiceGmail.js text: " " and replace with " "
6) save edited file and update archive with edited file gmServiceGmail.js then rename it back to XPI
7) reopen Firefox then drag this modified XPI file in Firefox this will open extensions install window're DONE

Credits: kzk0813 on April 12, 2012