0. Quit Safari (it might freak out if you mess with the Extensions dir while it’s running)
1. Open Terminal
2. cd ~/Library/Safari
3. mv Extensions Extensions.old
4. ln -s ~/Dropbox/Extensions .
5. Done! Fire up Safari and check it worked.

Note: I’ve not tested this myself.

First go to the dropbox folder where you want the extensions saved:
cd /path/to/dropbox/

Then make a soft link from your safari extensions folder to softlinkname inside your dropbox:
ln -s ~/Library/Safari/Extensions softlinkname

Now your items are synced over dropbox as soon as you add them to safari, now for the setup on the 2nd Mac:

Navigate to your safari extensions folder, and remove it (because you’re going to use the softlinked version instead)
Then open up the terminal and also navigate to the safari folder:
cd ~/Library/Safari

Then make a softlink from the folder with your extensions inside your dropbox to the Safari folder and name it Extensions to replace the one you removed:
ln -s /path/to/dropbox/softlinkname Extensions

And that’s it, you can now add extensions to either Safari copy and it’ll be automatically synced between computers without any further user interaction required. though probably not for those who are afraid of the terminal ;)
(Also, I’m not entirely sure how well it will do if you keep both Safari’s open at both computers at the same time.)

I hope that was a thorough and understandable explanation!

It’s pretty straightforward. Here’s what I did (your storage location within your Dropbox directory may differ):

$ mv ~/Library/Safari/Extensions ~/Dropbox/AppSupport/safari
$ ln -s ~/Dropbox/AppSupport/safari/Extensions Extensions

That’s it. Dropbox now owns the physical files and happily syncs them while your Safari install just knows to look there when accessing extension info.

That will create a symbolic link between your extensions directory and a directory in your Dropbox called “Extensions”. The two folders will sync (actually the latter is just a pointer to the former, but regardless) without you having to do anything.

cd ~/Dropbox
ln -s ~/Library/Safari/Extensions .

1. Move your Extensions folder (from /Library/Safari) to your Dropbox folder (/Dropbox by default)
2. Open up the Terminal application (/Applications/Utilities)
3. Type the line below

ln -s ~/Dropbox/Extensions/ ~/Library/Safari/Extensions

The third step creates a symbolic link (like an alias as MariusTH86 notes) so that when Safari looks for the Extensions folder, it is redirected to the shared folder on Dropbox, which will sync automatically.

On other computers, first move any extensions not on the first machine from the Extensions folder in Library/Safari/Extensions to the Extensions folder in Dropbox, then delete the folder in Library/Safari/Extensions (if you copy the whole folder as in step 1, you will overwrite the first lot of extensions. Then, type the line from step 3 this second Mac.

Hope this helps. It’s working so far for me.