I have just tested the add on with firefox on windows 7. The sliders seem to work as they are supposed to. Please note that they only work for conversions to .jpg. png is a lossless format and therefore quality cannot be reduced thus the sliders are grayed out.

In other encoders you might be able to provide something like a compression level. This however changes the amount of compression applied to the image, and does not affect the image quality. Only the filesize is reduced or increased at the cost of encoding and decoding times, the content of the file will be the same. Changeing the compression level is not available in the firefox build in encoder en thus not provided.