Would it be safe for me to use firefox and chrome on my xp desktop after April 8, 2014?

It will be as safe any other browser on that machine.

In addition to Avast, I suggest getting the free version of Malwarebytes and performing a manual scan once a week. Just kick it off before you go to bed.

Some additional steps you should take are:
1) run the free version of Spybot or Adaware to scan for adware on your system.
2) use a tool like CCleaner to remove things from your system which might be slowing things down.
3) run a registry optimizer to organize/clean it up (I use free software for this task, just search a place like download.cnet.com and you should find a number of them).
4) run msconfig and check your start-up for items being started at boot/login which don't need to be or that you don't want started then. That will shorten the boot/login.
5) go to www.blackviper.com and look at his guide to tweaking XP (and Windows 7/8 when you get there)

Another possibility to consider is this: Linux will run just fine on that system and it will perform better/faster than it does now. Some of the latest versions of Linux look and feel very much like Windows and are such that you should be able to sit right down and get going on it. I recommend Linux Mint with the Cinnamon desktop. It is perfect for non-nerds and scads safer than Windows. If you still need Windows, then you can run Oracle's VirtualBox (free) and install your XP within that.
Something else to consider is that starting at as low as $100 you can get refurbished machines from Newegg.com which come with Windows 7. They will all almost certainly out perform what you have, have more RAM and hard disk space, and have an operating system which is being maintained.

only their support is stopping