When you install two extensions that modify the same elements you can get a warning, as uBo with Ghostery, and keep using both.
Another case is Privacy Badger with uMatrix that needs the element to be untouched by one of the two extensions (in this case spoof HTTP referrer with Umatrix).
Atavic
All 2 comments
uM with Tampermponkey: https://github.com/gorhill/uMatrix/issues/778#issuecomment-298668580
Atavic
on 4 Jan 2018
The problem: Firefox has known problems when two extensions modify the same headers.
The issues appear because the final request that's outgoing from the browser is ruled by the last extension in the chain.
So, if you load e.g. uMatrix first and then CanvasBlocker, the outgoing request is ruled by the latter extension. Moreover, when you update an extension, it goes on bottom of the chain, so the outgoing may change.
Another issue is the loading time of an extension, see uBlock Origin wiki
Atavic
on 11 Sep 2018
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Most helpful comment
The problem: Firefox has known problems when two extensions modify the same headers.
The issues appear because the final request that's outgoing from the browser is ruled by the last extension in the chain.
So, if you load e.g. uMatrix first and then CanvasBlocker, the outgoing request is ruled by the latter extension. Moreover, when you update an extension, it goes on bottom of the chain, so the outgoing may change.
Another issue is the loading time of an extension, see uBlock Origin wiki