This will depend on each extension and how it was built. Some extension may consume memory even while not being used because they may load objects into memory as part of their startup code. Extensions with static objects or objects with static data members will consume memory too.
You have one great way to test which extension may be more problematic in terms of memory consumption while not being used.
- Disable all your extensions. Open
Firefox and note its memory
consumption under task manager on
Windows. Enable one extension at a time
and restart firefox. Note changes in
One last thing to take in consideration. Even if an extension does not consume (much, or any) memory while not being used, it may do so after it has been used. Firefox likes to cache objects for later use once these have entered the memory space. An extension may not be in use, but since it has just a few minutes ago, it's memory footprint (or part of it) may remain cached for the whole firefox session. As long as the cache isn't exhausted by other objects, it will stay there.
One last thought that occurred to me:
An extension is installed by default only to the current Firefox Profile. You can manage your extensions on different profiles with tools like Mr. Tech Toolkit, enabling and disabling extensions on a profile basis. As such, if you are concerned with memory, an alternative to manually disable and enable extensions is to create a few Firefox profiles. One with no extensions, another with all extensions and as many as you like with some but not all extensions. I'm imagining something like the following Profiles:
- Clean (no extensions)
- Developer (developer specific extensions)
- Social (social web & blog extensions)
- Full (all extensions)