I have two browser extensions that I use a lot. FicLab is a browser extension and works for both ff and AO3 and has multiple formats for downloading, you might have to tweak the layout as the default is indent + no spacing but there are other options. AO3 Series Downloader is also a browser extension that lets you download a entire series but it's all individual works and the OP is looking at options for a zip file download too OP twitter and links to install.