Linkswift is not malware. Its merely adware that got installed by you. Most firewalls by default do not block adware. If you want a pop up to block everything then set the firewall to ask. Clearly Avast is not at fault here. Linkswift is not malicious. Its just like installing the Ask Toolbar.
as stated above, Linkswift is not malicious… it is classed as PUP
PUP = not a virus / Possible Unwanted Program
LinkSwift is technically not a virus but a potentially unwanted program (PUP) that takes at the format of extension to hijack users’ browsers including Internet Explorer, Mozilla Firefox as well as Google Chrome.
Bundled with freeware ((video recording/streaming, download-managers or PDF creators), LinkSwift goes to your PC without your knowledge. And then, it will install LinkSwift Toolbar onto your PC; meanwhile, all of your browsers will be added extension—LinkSwift to change your internet experience. As a result, you will receive popup ads from LinkSwift in your search results.
PUP scan is default OFF in quick/full scan … but default ON in boot scan