I reinstalled myibay today. I use Firefox. When I used the trial version I would click on the roundel on the auction page in ebay and was able to place a bid. Today the snipe roundel appears in the same place but is inactive. How can I rectify this to enable me to snipe?

The Firefox add-on does not have any trials. It's completely free, no limitations.
I'm not sure what do you mean by "inactive". If you see it, you should be able to click on it. What happens when you click the "Snipe it" link?

Also, make sure you are using the latest version which is always available at the following location:
https://addons.mozilla.org/en-US/firefox/addon/5571

I appreciate that the Firefox add-on does not have a trial period. But myibay does!. Subsequent to buying the programme at the end of 30 days. it isn't working, even after clicking on the roundel on the ebay auction screen. How can I get it to work?

I think you are mixing many things together so it's confusing.

Firefox add-on is completely free. If you don't see "Snipe it" icon on the auction's page, you have to install it:
https://addons.mozilla.org/en-US/firefox/addon/5571

Myibay service is free. No need to buy it every 30 days or anything like that. It's completely free.

Myibay Windows application is the only part of Myibay which has 30 days trial. But it does not require to make payment every 30 days.
Even more, you are not required to buy it at all. You buy it if you want to support Myibay service or don't want to use the Web interface.
http://www.myibay.com/download/

The Firefox add-on has nothing to do with Windows application. These are two separate non-related products.