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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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No.

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:

  1. It's incompatible with a given version of Firefox.
  2. There is an error in the extension.
  3. The functionality could be prohibitive to certain users.
  4. It's only needed at certain times.
  5. The user explicitly does not want to use the extension.

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.

Consider:

  • Why would you load something that isn't by definition supposed to be loaded?
  • Imagine if you had a buggy extension that crashed as soon as it was loaded, and took the browser with it. You wouldn't see that behavior if the extension was disabled.

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.

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    Is there any documentation to back this up?
    – Travis Northcutt
    Aug 27, 2009 at 18:18
  • Not that I am aware of, but I'm willing to bet that this is the behaviour, especially when considered from a programming perspective. See my edit for details.
    – EvilChookie
    Aug 27, 2009 at 18:37
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    An extension can also be disabled because you disabled it. I disable extensions that I haven't used in a while all the time, while leaving them installed, just so when I do need them later, they're there but not slowing down the browser in the mean time.
    – Will Eddins
    Aug 27, 2009 at 18:52
  • @Guard: Thanks for reminding me. There was a 5th one, but I couldn't figure it out.
    – EvilChookie
    Aug 27, 2009 at 18:58
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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.

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  • True, but it is conceivable that disable extensions could still cause some performance loss, especially with numerous extensions disabled but still installed.
    – Travis Northcutt
    Aug 27, 2009 at 18:17
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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.

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