Hi This may not be a strictly Firefox issue, but I will see if I can help. I recommend using the tools installed in Windows to defragment the drive and clear up any temporary files. Especially if you have recently upgraded to Windows 10, there may be an awful lot of "litter" lying around that could be tidied up. After that, you may find that there are features of the Avast package that could be regarded as "bloat". I should be careful here as I do not want to advise you to turn off important features, but essentially it could be the case that the very software that is protecting you is slowing down your PC slightly. I hope this helps, but if not, please come back here and we can look into a different solution for you.