FB2 Reader is described as 'FictionBook (.fb2) e-book reader addon for Firefox' and is an app in the web browsers category. There are three alternatives to FB2 Reader for Windows, iPad, iPhone and Linux. The best alternative is SumatraPDF, which is both free and Open Source. Other great apps like FB2 Reader are Panels - Comic Reader and SourcePDF.
FB2 Reader alternatives are mainly PDF Readers but may also be Ebook Readers or PDF Editors. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of FB2 Reader.