Flowering rose,I would suggest that the reason you could grow nothing under fir trees was due to lack of water and shade rather than acidity. Fir trees are very shallow rooted and take all surface moisture from the soil.
Vivienne Horner, I would take the advice given by obelixx above and add that you should only mulch when the soil is moist - keeping the moisture in rather than keeping it out, esp in the dry period we're having at the moment (in my part of the world, anyway)