From MozillaZine Knowledge Base
If startup is slow, these solutions may help.
- Defragment your hard drive (accessed in Windows as "Disk Defragmenter", found in "All Programs -> Accessories -> System Tools" in the Start Menu)
- Clear your cache, cookies, and history in Firefox by pressing "Ctrl+Shift+Del" and check the applicable options, or in the Mozilla Suite preferences (under "Advanced -> Cache" and "Privacy & Security -> Cookies -> Cookie Manager").
- Clean out your bookmarks.
- If the other solutions fail, the standard diagnostic may be able to fix the problem.