Over the last few weeks, my Firefox browser has been extremely SLUGGISH, especially when loading up.
It has taken around 30 seconds to load, when it used to take around 2-5.
So I went looking for a solution, whilst also looking up why I had 20000 files in that folder.
Little did I know that the solutions were one and the same!
Below is a guide I have put together to deal with these problems.

Important: I recommend reading the whole of the guide carefully before starting

Close Firefox completely by clicking File --> Exit.
Open the Run dialog box by clicking START --> Run, or pressing Windows-R.
Type or copy the following into the dialog box, without quotes - "%APPDATA%\Mozilla\Firefox\Profiles" and press enter.
Click the folder there, which is named randomly.
If you have more than one profile (folder), follow these steps for each one.

Important: If you are at all unsure, or want to be careful, please copy this folder (or these folders if you have 2+ of them) elsewhere first, so you have a BACKUP.

Look for all the cookies-1.txt, cookies-1.txt etc files, and delete them.
On mine, I had cookies-1 to cookies-9999.
Be aware that this may take a while if you have lots of them and your system may appear to freeze - leave it alone for a few minutes.
Press F5 to refresh the folder.
Then do the same for prefs-1.js, prefs-2.js and so on - I had 8600-odd of these.
Again, be patient.

Important - make sure you do not delete "cookies.txt" or "prefs.js" - these are your preferences and cookie files

Press F5 to refresh the folder.
Check again for any of these files.
Now right click prefs.js and cookies.txt, choose properties, and ensure that the read-only attribute is unticked.
If you like, you can also delete bookmarks-x.html and bookmarks.bak - these are your bookmark backup files - and the files within the folder called "Bookmarkbackups".

Important: Make sure you do not delete bookmarks.html - this is your bookmarks file.

Close the window, and type or copy the following, without quotes, into the Run dialog box "%APPDATA%"
Right click the folder named "Mozilla" and ensure Read Only is unticked. When it asks if you want to apply the change to just the folder, or all subfolders and files, choose all of them.
Close the window.
Now empty the Recycle Bin and defragment your HDD.

I hope this helps some people out, as it helped me out.
Calum.

P.S. If you use this guide, please let me know how it goes.
Thanks.Odd -- I don't have these cookies-*.txt files:

Directory of C:\Documents and Settings\Timothy\Application Data\Mozilla\Firefox\Profiles\f4mhvkkd.default

04/21/2007 08:20 AM <DIR> .
04/21/2007 08:20 AM <DIR> ..
04/21/2007 07:41 AM <DIR> adblockplus
04/20/2007 04:29 PM 100 blocklist.xml
04/21/2007 07:41 AM <DIR> bookmarkbackups
04/20/2007 10:22 PM 316,176 bookmarks.bak
04/20/2007 10:22 PM 316,176 bookmarks.html
12/21/2006 11:47 AM 40,456 bookmarks.html.sbsd.bak
12/07/2006 04:37 PM 427 cats.smx
04/21/2007 07:41 AM 65,536 cert8.db
02/24/2007 01:42 AM <DIR> chrome
03/20/2007 09:51 PM 132 compatibility.ini
04/21/2007 07:41 AM 149,244 compreg.dat
04/21/2007 08:17 AM 22,577 cookies.txt
04/20/2007 09:05 PM 206 downloads.rdf
04/20/2007 10:05 PM 12 dta_history.xml
04/21/2007 07:41 AM <DIR> extensions
04/21/2007 07:41 AM 1,458 extensions.cache
04/21/2007 07:41 AM 1,663 extensions.ini
04/21/2007 07:41 AM 21,826 extensions.rdf
04/20/2007 10:22 PM 292,489 formhistory.dat
04/21/2007 08:20 AM 7,339,881 history.dat
04/20/2007 05:34 PM 9,301 hostperm.1
04/21/2007 07:41 AM 16,384 key3.db
04/21/2007 07:41 AM 77 kf.txt
04/21/2007 07:42 AM 23,336 localstore.rdf
02/18/2007 11:53 PM 2,925 mimeTypes.rdf
04/21/2007 07:41 AM 0 parent.lock
04/13/2007 01:03 PM 1,054 persdict.dat
04/21/2007 07:46 AM 35,785 prefs.js
10/11/2006 01:05 AM 3,287 search.rdf
11/19/2006 09:01 PM 2,048 search.sqlite
11/19/2006 09:01 PM 16,384 secmod.db
04/21/2007 08:20 AM 10,764 sessionstore.js
03/24/2007 11:33 AM 2,961 signons2.txt
12/07/2006 04:37 PM 159,288 smiley.smx
12/07/2006 04:37 PM 5,455 smxCat1.smx
12/20/2006 11:01 AM 6,378 smxCat14.smx
12/11/2006 10:04 PM 19,876 smxCat15.smx
03/10/2007 12:05 AM 11,081 smxCat2.smx
12/07/2006 04:37 PM 3,058 smxCat20.smx
12/21/2006 07:57 PM 5,444 smxCat21.smx
03/10/2007 12:04 AM 933 smxCat22.smx
02/06/2007 07:51 PM 3,334 smxCat24.smx
12/11/2006 10:04 PM 1,869 smxCat26.smx
12/22/2006 10:21 PM 2,914 smxCat4.smx
03/26/2007 11:26 AM 1,959 smxCat5.smx
12/11/2006 10:04 PM 2,512 smxCat9.smx
04/17/2007 09:08 PM 23,527 stumblerating2333130
03/06/2007 10:42 PM 69 stumbletags2333130
04/21/2007 07:41 AM <DIR> StumbleUpon
02/22/2007 09:43 AM 0 stumbleurls0
04/21/2007 07:42 AM 253 stumbleurls2333130
04/21/2007 07:43 AM 7,582,720 urlclassifier2.sqlite
04/21/2007 07:41 AM 94,666 xpti.dat
48 File(s) 16,618,001 bytesGood for you.
Let me add, then, that this will not apply to all people.
It is a BIG, and happens to some and not others, caused by the relevant files being set to read only (which can, apparently, happen without the user's knowledge - I didn't change them to be read only).
Also, the family PC had no prefs-*.js, and had about 5 or 6000 bookmarks-*.html and about 700 cookies-*.txt.
Very different from mine.
So apparently it varies greatly too.