1. Are we the only people who think of PHP+Firebird as producing apps just like good old Delphi+Interbase?

We are not the only ones There are many that  use php and firebird in production (drivers are quite stable) and also there are many frameworks to work with : laravel , yii ... https://packagist.org/packages/marcha/laravel-firebird-support

We need a hhvm driver but that needs to be written from scratch 

2. Do people [on any stack] bother to cache query results as a matter of course?

Yes we do at least in Rails, Django we use memcache and nginx as frontend for static content  but you need to benchmark and measure if is worth the effort 

3. If we've got servers with 30GB+ of database that are all OS-read-cached and PHP/Apache is happily sitting in < 1GB of memory, should we even be worried about op-code caching? Why is the PHP world obsessed with caching and load balancing?

If you have millions of users then any improvement it helps , but you can install op-code caching in any php version  here is an article for tunning a webserver for php 

http://2bits.com/articles/high-performance-drupal-with-apache-mpm-worker-threaded-server-and-php-fpm.html

It can be switched easily with nginx if you don't need .htaccess

4. I've been "happy" with php for 14 years, have I been mad and should we have been using Zend Server all along? [I respect your opinions more than any other forum even though it's nothing to do with firebird!]

I usuall use the stock php with opcode caching for sites with thousands of concurent users and if in the future maybe hhvm