Do disabled Firefox extensions make Firefox slower?
I know that extensions make browsing slower, but what about extensions that are disabled and not in use?
Firefox doesn't load extensions that are disabled. If it's not loaded, it causes no loss of performance.
An extension is disabled for one of these reasons:
Loading a disabled extension can cause errors, or undesired / unsecure behavior. It makes sense that the developer would not load the disabled extensions in any fashion for these and performance reasons.
As mentioned in the comments, I have not sought out any official documentation regarding this, so take the answer with a pinch of salt. However, any other behavior concerning extensions would detract from the user experience, which I bet is the last thing that Mozilla wants to do.
From the addons.mozilla.org FAQ:
The final sentence there implies that disabling an extension will prevent it from having any noticeable effect on Firefox.
Firefox will still look for updates of disabled add-ons. This probably causes some additional background work and delay during start-up, but hardly noticeable unless you have dozens of add-ons installed.