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The current Norton Toolbar extension is built using legacy technologies that will not be supported by Mozilla after Firefox version 57.
Reads much better as " ... technologies that Mozilla will no longer support after ... "
Once, you update to Mozilla Firefox version 57 ...
One should delete the ungrammatical comma, "Once you update ..."
... all extensions which are built using legacy standards will be ...
Brevity suggests "... all extensions built using legacy standards will be ..."
This new extension is built on technologies that is supported by Mozilla ...
The noun "technologies" being plural, the phrase should read " ... technologies that are supported by Mozilla ..."
But again, brevity: "... extension is built on technologies supported by Mozilla ..."
... and implement toolbar less extension model.
Made clearer by inserting the missing hyphen: "... implement toolbar-less extension model."
<kidding>I'm available as a copy editor!</kidding>