I'd say it's one of two things:
1. An extension you have installed tracks the websites you visit and sends that to a third party.
2. Monarch Airlines has scripts that track your visit and sends that to a third party.
In this case it looks like #2, because checking the source code for http://www.monarch.co.uk
shows lots of scripts, some of which are RedEye Tracking, Google Analytics, Google Tag Manager, DoubleClick (owned by Google). It's very likely that your visit was shared with an advertising network that displays ads on a wide variety of websites.
There are a couple of ways to prevent this:
1. Install an extension such as Privacy Browser
, although I find this can sometimes break images and styles on a page.
2. In Firefox go to about:config and search for "tracking". Then double-click on "privacy.trackingprotection.enabled" to change the value to "true".
You might want to delete cookies as well to get rid of sites that are already tracking you.