Do you ever cleaned up your machine with a cleaner (like CCleaner) and defrag it (free - Defraggler, Puran)? How many things do you have at Start Up? Anything that is not necessary, you can “Disable.”
How much memory on your machine?
Is your machine running normally otherwise? Do you have this problem in IE? FF has been adding more security features and depending on your add-on’s, it may be slowing things down. Do you have add-on’s that you don’t need? Are you using Tor?
Have your scans (both Avast and MBAM) been OK? BTW, the XP Windows Firewall is worthless. You may want to look into a free third-party FW for 2-way protection. Now you have 1-way protection. OA Free is light on your system, but you can also do a search here and find others compatible with Avast (e.g. Outpost, Comodo, etc.). But a lot depends on your memory and if you clean it or not. XP’s need to be cleaned and defragged.
Since Firefox has been my browser of choice for some considerable time and I have been using avast! for over ten years, I haven’t experienced that problem.
For some time now firefox has been opening in around 12 seconds. This actually improved somewhat in FF 31.0 and now opens in 11 seconds.
I have 28 add-ons and these actually have an impact on loading, so as SafeSurf mentions check your add-ons for redundancy. I currently have 9 pinned pages that will start loading once firefox has loaded/opened.
You don’t say what this 30 seconds covers, just firefox being displayed (loaded/open), as in my own example or also the loading of any pinned pages ?
The Web Shield only gets involved when you are loading pages, so presumably you too must have some pinned pages, again if you have look for redundancy, e.g. do you need all of them to load on firefox startup.