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Gmail manager is one of the most useful addons to quickly check your Gmail mail account via Firefox browser.

But from time to time, as Firefox updates itself, there are compatibility problems . Gmail manager version 0.5.7.4 , which is the last officially released version, can not login to the account.

The fix is either:

1. use new experimental version 0.5.7.5
2. If a new experimental version is not to be found, uninstall Gmail manager and temporarily  install a previously released version, until a fixed new version is published.

Here is a page that contains all the Gmail manager versions, from the newest to the oldest:
https://addons.mozilla.org/en-US/firefox/addons/versions/1320

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