Security of Firefox extensions such as Mailvelope

I am asking these question because I am not very familiar with javascript or the Firefox plugin/extension system. The specific context is the Mailvelope pgp extension but the questions are general and would apply to all extensions.

-Can another extension access memory in use or previously used by another extension?

-Can another extension act as an MITM keylogger? Can javascript from another page/tab that is open?

-Can another extension modify the code of another (eg, on disk)

TIA