How to edit a Firefox extension?

I want to change a Firefox extension. What I've done is found it in the Firefox profile folder, renamed the .jar to .zip, unzipped it, made the changes in a .js file (I didn't make a mistake in JS code), zipped it back and renamed to .jar. So now the button to start that extension has disappeared from the menu in Firefox :) What is the proper way to bring minor changes to a Firefox plugin?

Update: I've tried to make the changes minimum to test if it'll开发者_StackOverflow中文版 work: I've replaced

stepsVariations.push(new WTRStep("clickButton", {label: _oTarget.alt}))

with

stepsVariations.push(new WTRStep("clickButton", {label: _oTarget.alt+"test"}))

still the same

Update: I've tried the .xpi thing, still the same - not even a button in the menu.

I suggest getting the .xpi from addons.mozilla or the original site. Rename it to zip and do your changes, then rename it back to .xpi and open it with Firefox, it will be installed over the old one. See if it makes a difference.