Please update to Firefox 25, 21 is old and out of date. Update Firefox to the latest release

A tad faster but still slow compared to IE 8,9,10 where it starts instantly.

Note that your System Details List is showing a very old Flash 9.0 r45 version.

1. Shockwave Flash 11.9 r900
2. Shockwave Flash 9.0 r45

You should see the path of this plugin on the about:plugins page (open via the location bar) and delete this file.
See also:

• http://kb.mozillazine.org/Flash

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

It is also possible that your security software (firewall, anti-virus) blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.

Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/kb/Server+not+found
• https://support.mozilla.org/kb/Firewalls

• https://support.mozilla.org/kb/fix-problems-connecting-websites-after-updating

Also, please note that IE will almost always feel like it starts up faster than Firefox, because there are portions of IE that are bundled into the rest of Windows, and start when the computer starts,so it has less work to do when it actually is opened. Firefox can't do that, meaning it might feel somewhat slower.

Is this path and file that I should delete manually?

Shockwave Flash

   File: NPSWF32.dll
   Path: C:\Windows\SysWOW64\NPSWF32.dll
   Version: 9.0.45.0
   State: Enabled (STATE_VULNERABLE_UPDATE_AVAILABLE)
   Shockwave Flash 9.0 r45

Yes, that looks like the correct file.
The current releases have the version number in the name of the NPSWF32_11_X_XXX_XXX.dll file.

I deleted this file, but startup is still slow at 10 seconds compared to IE9 which is like lightning

The first start can take longer because the files aren't in the Windows disk cache, but subsequent starts should be a lot faster.
Note that it is also possible that security (anti-virus) software is causing issues by checking/monitoring read and write file actions.

Each and every fresh start is the same it takes 10 seconds for Firefox to start up even with the anti-virus disabled.

There are a few things you can do:

Scan for Malware, Troubleshoot Firefox issues caused by malware (note that scans with your regular anti-virus aren't enough)

Update your graphics Driver, Upgrade your graphics drivers to use hardware acceleration and WebGL

Ensure Windows is 100% up to date

Don't use things like System mechanic as they actually cause more damage to your computer than they help

Run some general clean up tools like Disk Cleanup, Disk Defrag, etc. Then restart your computer. Note that after running Disk Cleanup Firefox will take a bit longer to start but subsequent starts should be faster.

Note that we are starting to go out of the realm of Firefox troubleshooting here and more into general computer maintenance, and there are many variables on your machine that Firefox has no control over. Depending on how your computer is set up, how many programs you have running, the resources available, a 10 second startup may be the best you can do. You can take my advice above to try to improve it, but not that it may not significantly improve Firefox's startup Speed (IE will always start faster for the reasons above).

I have an i7 MBP with a SSD and 16GB of RAM and Firefox takes 2-3 seconds to start with that beefy of a system.

Also, fresh starts (right after you turn your computer on) will ALWAYS be slower no matter what app your are running.