Why do add-ons become incompatible when upgrading firefox

Each addon in Firefox has setting which specifies which versions of Firefox it will work with (the presumption is that the author has tested their addon with the versions mentioned). Historically, addons typically claimed compatibility with a major release of Firefox (e.g. 2.X) but then there were problems with differences between 3.0 and 3.6 so addon authors got more specific and only claimed compatibility with particular major and minor versions (e.g. 3.6.X). A range of versions can be specified (e.g. 2.X -> 3.6.X) but an upper limit has to be given and it can't be an unreleased future version of Firefox if you want to distribute your addon via the Mozilla site.

Now that "major" versions of Firefox are being released every 6 to 8 weeks (we've seen versions 4.0 through 6.0 since March) addon authors just can't keep up and more and more are falling behind.

You will occasionally see a message on the Chrome "Extensions" page saying that a particular extension needs to be updated to work with the current version of Chrome but, since updates are done automatically in the background, it's hard to catch one in this state.

I hear you! I ran Firefox today for the first time in ages just to use one addon - it then said an update is available - I let it do this, and then, all my addons got disabled!

Firefox updates do not actually mark individual addons as incompatible per se, it is the addon authors not stating that they are compatible which disables them.

You can use the add on compatibility reporter which will allow you to report addons which are not "compatible" with your Firefox version.

As for why Chrome doesn't do this - I think it is subjective and you will get different answers - IMHO, it is designed in a more traditional way - it allows all add-ons and if anything breaks, the onus is on the user to find the error where as by disabling until they know it is safe, Firefox may annoy the average user, but, is a safer solution to the end user.