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:
It's incompatible with a given version of Firefox. There is an error in the extension. The functionality could be prohibitive to certain users. It's only needed at certain times. 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.
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."
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.