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?
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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:
Can add-ons make Firefox slower?
In most cases, add-ons do not cause a perceivable slowdown in Firefox. However, since they are applications some may affect the performance of Firefox depending on your system configuration. If you suspect that an add-on is affecting the way Firefox runs on your machine try disabling it.
The final sentence there implies that disabling an extension will prevent it from having any noticeable effect on Firefox.