Webshield apparently fudges around with Youtube in a way that builds after 2014-10-31 don’t load up videos due to Cross-Origin Request Blocked errors.
https://bugzilla.mozilla.org/show_bug.cgi?id=1093466
What sort of problems you are facing! I highly doubt it that there’s bug in Avast 2015. Nightly versions of any program is considered very unstable and it won’t be supported by any antivirus program can cause system errors on your computer so make sure to back up before you install any other nightly programs. It will only be supported when that firefox version you are running becomes officially released.