It would help to know a little about your system spec, e.g. OS, CPU, RAM, etc.
avast scan files that are being loaded to provide protection, firefox loads lots of files and I personally can't see why you would want a browser to load before your AV. Check the programs that load on boot and prune those not essential, such as media players, etc. that aren't required until media files are played.
How many files are reported as being scanned (standard shield, Scanned count:) once avast's icon stops rotating ?
What is standard shield's sensitivity set to ? The default is Normal higher than that and more files are scanned.
Do you have any other security software running on boot ?