Do disabled Firefox extensions make Firefox slower?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

Yes, if the extension blocks things that slows down Firefox. E.g., if you have an extension that blocks Javascript and you visit a site that use a lot of JS, so much that it slows down Firefox, disabling that extension might slow down Firefox. Of course, blocking Javascript (or something else) might break a site. In other words, an extension that blocks third party JS used for ads might speed up Firefox.