Mozilla have now (12th May) released a new version (V1.0.4) which covers the problems with earlier versions. The advice is to download and install it ASAP.
Errors in the earlier versions are discussed
here Apparently they are not going to release details for a few days.
I've just done it and it seems to work fine.
If you have the appropriate box checked (Tools/Options/Advanced/Software updates) you will get a small red triangle in the top right-hand corner of the screen, next to the circle of grey dots, when an update is available. Click on that and you will initiate the download. When you have installed it you may find that the triangle is till there. Click on it again and it will tell you that there are no new downloads and it will then disappear(!).
GG