If Firefox is slow, try following:
If you refresh Firefox, the history, tabs, bookmarks, passwords, formuladata & cookies will be kept.
Enter about:support in the address pane and click on Refresh Firefox (Firefox restaurieren).
Normally the installed add-ons will be active when you start Firefox. In case of Flash or Shockwave you should change this configuration to ask to activate.
If you have done the above steps, and Firefox doesn't speed up, so should check, if Firefox performs better, when you disable Javascript. To do so, follow these steps:
Hint: A lot of webpages doesn't function properly, when javascript is disabled.
When Firefox performs better when Javascript is disabled, so you should install the Add-On NoScript. But, especially at the beginning, you have to allow javscript for every (wanted) webpage.