Firefox Lags Really Bad On Startup

Reinstalling Firefox often does not fix this kind of startup hang because your profile (extensions, prefs, cache, session data) is kept across uninstalls. That makes @GaryKing's profile suggestion the right first test: a clean profile rules out corrupt settings or a misbehaving add-on. @Laser's point about tuning tools is also relevant — they can help by changing hidden prefs, but they can also hide the real cause, so proceed with backups and caution.

Quick troubleshooting order to try now (easy → more invasive):

• Start Firefox in Safe Mode to temporarily disable extensions and themes: run firefox -safe-mode. If startup is fast, an extension or theme is the culprit; disable them and re-enable one-by-one to find it.
• Create and test a new profile with the Profile Manager: run firefox -ProfileManager (or -P) and make a fresh profile. If the new profile is snappy, copy over bookmarks only and leave old extensions/settings behind. Back up the old profile folder before removing it. Typical Windows profile locations:
  C:\Documents and Settings\<username>\Application Data\Mozilla\Firefox\Profiles\ (XP) or
  C:\Users\<username>\AppData\Roaming\Mozilla\Firefox\Profiles\ (Vista+).
• If Safe Mode/new profile don’t help, check Task Manager while the lag happens. High CPU or disk I/O points to plugins, antivirus on-access scans, or disk problems. Temporarily disable real-time AV scanning (or exclude the Firefox profile/cache) to test.

Extra checks: disable plugin-heavy content (Flash/Java), stop session-restore of many tabs, clear cache/history, and try a new Windows user account (isolates a corrupt Windows user profile). Tools that bulk-tweak prefs can be useful, but prefer targeted fixes first and always back up bookmarks and the profile folder before deleting anything.