The one and only add-on you will need in my opinion is Gmail Notifier! You can install it from here - https://addons.mozilla.org/en-US/firefox/addon/gmail-notifier-restartless/
It is very light and is working flawlessly for several months now. You can install it restartlessly (it is restartless). and use it for your gmail accounts. Here are some advantages of this nice addon:
1. It has 66 positive reviews! Read them here - https://addons.mozilla.org/en-US/firefox/addon/gmail-notifier-restartless/reviews/
2. You can configure checking of new e-mails in seconds. For example 15
3. It displays the arrival of new e-mails on your desktop! So you do not need to have the gmail page open! Also - if you are watching a movie - it will also notify you and you will see a pop-up notification window!
4. You can play an alert sound on every new e-mail arrival. You can choose which alert sound to play (from your PC)
5. Choose the color theme of the add-on - currently there are Blue and Grey
6. It is completely FREE and is working just fine! Love it! :-)
Gmail Notifier (restartless) is an open-source project that notify you about incoming emails from all your Google Mail accounts and labels.
1. No requirement to enter your credentials.
2. Multiple account support
3. Multiple label support
4. Low bandwidth usage by using RSS technology
5. Mark as read, report span, trash or archive massages right from Firefox toolbar
1. To add/remove the toolbar icon right click on the Firefox toolbar and select "Customize..."
2. To change settings go to Addon-manager -> Gmail notifier -> Options
3. Get label-specific notifications: https://add0n.com/gmail-notifier.html
Left click: Opens Gmail
Middle (or Ctrl+Left) click: Refreshes all accounts
Right click: Account selections
Need more help:
Check this page for more instructions: https://add0n.com/gmail-notifier.html