When I click on my desktop icon to connect to the internet (Mozilla Firefos) the start page comes up OK, but then a new tab opens up and I'm presented with various boxes to click on, eg, tesco, facebook etc
I've no idea why and how this happened and would like to get rid of it.
Have gone into control panel, but there doesn't seem to be anything untoward in there.
I think that's something to do with the latest version of Firefox: it presents the pages you've most recently (or most often) used, so you can click on one if you want. Quite convenient if you're so inclined; if not, just type in the URL you do want, same as normal.
This happens on OH's computer, but not on mine, I think because I've set it to clear my browsing history every time I log off.
If it's simply occurring when you actually open a new tab, then the 'about:config' procedure, in Jno's link, will fix the problem.
However if a new tab is being opened without you calling for one, then it suggests that there's malware on your computer. Download, install and run the free version of Malwarebytes Anti0Malware to get rid of it:
https://www.malwarebytes.org/antimalware/
(When installing,take care to deselect the option to take a free trial of the 'Premium' version).
Based on your description, it's likely that your computer has been affected by adware programs or PUPs that are good at displaying ads in new tab.
You can go to uninstall all programs that you don't know in your control panel and then go to remove all unwanted extensions in your browsers to see if it can fix your problem.