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This is my top useful and popular plugins for Firefox browser, which I use. This good plugins don’t slow down the browser and ease of use.
I prefer it instead of the bookmark manager. Support groups, automatic updates. So many useful settings.
Youtube High Definition
All youtube videos is always opened in HD. Very convenient.
Adds a sidebar on the left like in Opera. It could be quickly shown or hidden.
There you can find downloads, plug-in and settings manager, history and bookmarks, information on the page, etc.
Simply irreplaceable thing.
Todoist: To-Do list and Task Manager
Very useful todo list with synchronization and scheduling.
Block all the advertising in web pages.