I don't have Firefox, so I can't answer this question specifically.
However, you can use Safari's Reading List.
Tap on the Share Sheet and choose Add To Reading List from the list below the icons. This will save the current page for reading offline.
When you want to read the page, tap the book icon at the top left, then tap the glasses icon in the sidebar that appears. All you saved pages are there. Just tap the one you want to read.
Safari's reading list is enough for me, but it is fairly basic. If you want more powerful offline article saving and reading features you can find many app tailored for this. Since they use the share sheet, they are not browser specific and should work with Firefox. I hear often hear good things about Pocket and Instapaper, however I don't use them myself, so please do your research before buying.
Side Note:
I use Reeder by Slivio Rizzi as my RSS reader (Feedly for it's service). It also has a read later feature. I've played with it a bit and it works well on the iPad. If you want both a read later and RSS reader, it's a pretty good choice.
