It's mainly on Firefox that the ads are disappearing and leaving a blank space.

This is usually blamed on lack of appropriate inventory. I have just checked my site and found one with a blank grey box. I hit F5 and it acquired an ad.

Seen several more and suddenly wondered if it is because I have disabled Flash.

wondered if it is because I have disabled Flash.

I've been seeing a lot more Flash ads in AdSense appearing on my sites. You can turn off animated ads (Flash, animated .GIFs), which might be a good option if you have a lot of visitors that have Flash disabled.

Hi just for whatever reason some of my adsense ads are not showing adverts instead showing a blank space.

i get that sometimes as well and it is always because of flash.
every time flash releases an update safari stops showing the ads for security reasons (because my flash is now out of date).
as soon as i update it they start appearing okay again

If you want to find out how many times you're NOT serving up an ad, then put a unique image on your site (even if it's just a blank white gif), specify it as the backup in your AdSense code, and then see how many times the image is served.