Thousands of Firefox add-ons already work on Firefox 4
... But you can't make an omelette without breaking a few eggs -- and Firefox 4 is definitely a better omelette, as it were. As it turns out, plenty of developers have already begun adapting their extensions to work with the new version. Mitch Keeler of Firefox Facts points out that there are already at least 1,000 (the maximum number of results which the add-on site search will return) extensions which work with Firefox 4.
That's a pretty small chunk of the total you'll find on the add-ons site, but consider how many haven't been touched in years. The site is rife with outdated extensions and at some point they've got to fall by the wayside. More popular and actively developed extensions will update in time for the release -- so the vast majority of Firefox 4 users won't have to worry about breakage.
The final release is still a ways off, of course, so there's plenty of time for developers to make the necessary changes. If you're getting impatient, you can always follow Erez' example and offer the developer a little financial motivation to get the ball rolling!