1) If you have a lot of browser tabs open in firefox, you can configure it to open your tabs from your last session on startup, and stuff like that: in Firefox Preferences > General and Firefox Preferences > Privacy, you'll find the options you want (this will save your skin from crashes too). You can read PDFs in Firefox too (and thus get the but there are probably better PDF viewers you'll find.
2) As music players go I like Clementine for looking after my music library, it has all sorts of options for hooking into online services such as Grooveshark and Last.fm... You could also check out Miro at http://www.getmiro.com/ or in the Ubuntu Software Centre. The USC has an old version (Miro 4), but Miro 6 is now out.
3) The default image library software is called Shotwell, and doesn't have many tools; the GIMP is a much more feature-filled editor. You can get version 2.6 in the Software Centre, however to get the latest version (2.8) you will need to type these simple instructions in a Terminal :
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
sudo apt-get update
Source and more information: http://www.webupd8.org/2012/05/gimp-...lable-for.html
sudo apt-get install gimp