Rated 5 out of 5 stars

@KFM
600+ tabs is ALOT. I have only had at most 200.
Heres some things that could help.

Use profiles This:
It's best if you did this right now to get a firefox with no tabs and still have a version with tabs.
Try splitting firefox into different profiles. I have 2 profiles. One for my browsing and one called my "Firefox Lite" which is only allowed a few tabs open. This way I have a browser that will load quickly.
Read this guide
http://kb.mozillazine.org/Profile_Manager
Here's my icon for firefox "Lite"
http://img704.imageshack.us/img704/3513/firefoxlite35.png

Backup Tabs:
A way to backup your tabs is to install "Multiple tab handler" addon. Click on your first tab. Go all the way down to your last tab. Hold down the "Shift" key and click on the last tab. all your tabs should be highlighted. Right click and choose "Bookmark selected tabs to new folder."

TooManyTabs - addon
Gives you a bar like the quick bookmark bar, but it has several rows and you only see 1 at a time.
The bookmarks get deleted when you click on them and they then become tabs. When you bookmark the tab it will be closed. This sounds a bit like what you want.
You have to bookmark each page at a time.
OR, you could send all bookmarks to a new folder(Read Backup Tabs, above this) and then move the bookmarks to the "unsorted bookmarks > TooManyTabs"

Sort bookmarks by name:
I just organized my bookmarks and got them down from 380 to about 230. Some things were bookmarked twice, so go into "organize bookmarks" and right click on a folder and choose "sort by name".

Xmarks - addon:
use xmarks to synchronize your bookmarks between profiles. I found it really helpful with profiles as I can keep the same bookmarks and the profiles act the same way, so I don't loose much when choosing my "Lite" firefox.

Finally:
Close tabs that you don't need. Sounds obvious, but I keep tabs open that I don't need and some that I think I need. I have sorted out my tabs and now only have about 40 out of 160 open{spread out over the 2 profiles.}.

I know that you will be clinging on to some tabs, but if you doubt that you will ever use them, bookmark(and delete if never used) or just close.
You are probably a procrastinator. Someone who leaves things until the last moment. I'm one too, and the best thing that helps me is to "Just start for a minute". Just start cleaning up your tabs for 1 minute and you will soon be able to take a good amount of time to clean them up.
Good Luck!

This review is for a previous version of the add-on (0.8.2009122103).