Firefox keeps getting a pop up everytime i turn it on. I couldnt get a screenshot so i copied the log Avast
19:13:51 Network Shield: blocked access to malicious site https:// 54.200.231.121 ([54.200.231.121]:443) [ C:\Program Files (x86)\Mozilla Firefox\firefox.exe
I have attached logs below
What version firefox do you have ?
Based on the blocked address I would say that this are the recently introduced ads on the new tap page from Mozilla.https://forum.eset.com/topic/3608-mozillacom-address-has-been-blocked/
33.1
Could you run Firefox in safe mode and see if the alerts still occur https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
The alerts don’t always pop up usually when I open a new tab
Are you using tiles ? https://www.mozilla.org/en-US/firefox/tiles/
Nope its just a blank page when i open a new tab
That could be because Avast is blocking the tiles
How do I turn it on/off?
You can turn off Enhanced Tiles by clicking on the gear icon on the top right corner of the new tab page, and selecting Classic to show only unenhanced History Tiles or Blank mode, which turns the Tiles off.
Ok ive changed it to blank mode, Am I actually infected with something though?
No it is firefox putting adverts in your new tab based on where you have been before… I know that gives me some concerns, not that I use firefox
Time to switch to Chrome then I guess, so my systems not infected?
Nope there was nothing evident 
Thanks for your help Essexboy
My pleasure