Most firefox addons dont even have the permissions needed to change anything a website could observe.
Very strong disagree, I have seen and used many very widely used extensions that manipulate the DOM, which IMO satisfies your criteria of “something that can be observed” i.e. by javascript with a fingerprint tracker like creepjs.
Some examples:
-
ad blockers (uBO/uMatrix/etc.)
-
color/theme management (dark reader/dark theme/Stylish/etc.)
-
custom mouse cursor managers
-
page translators
-
addons serving in-browser ads
-
userscript managers (grease/tamper/violentmonkey etc.)
-
privacy blockers (CanvasBlocker/JShelter/etc.)
-
site-specific UI improvements (RES, SponsorBlock, youtube/SNS tweaks)
All of these can be detected and included as yet another bit of data that a unique fingerprint can be built from.
“Sent from my iPhone”