This can be caused by several different things, the most important:
- Hardcoding: some add-ons hardcode calls to Firefox in its compiled state. Since Pale Moon is a different binary than Firefox, this kind of coding will never work for any build that is not exactly Mozilla Firefox's distribution.
- Naming: some add-ons or external programs expect the program to be called "firefox.exe" - a simple problem with a simple solution for the add-on programmers to implement.
- Use of components not present in Pale Moon: If an add-on tries to use components that are specifically excluded from Pale Moon (like accessibility features, ActiveX, parental controls) it will fail.
The following few add-ons have been reported as being problematic:
- RoboForm
- KeyScrambler