Here is a free addon to your Firefox installation. I’ll quote from the site, and, by the way, why not give the author the translation help he asks for:
EPUBReader is a Firefox addon which let you read ePub-files just in the browser. You don’t need to install additional software!
If you click on a link to an ePub-file, you are normally prompted by the Firefox “save as” dialog. With EPUBReader installed, the ePub-file is downloaded, processed and directly displayed ready to read!
An ePub-file is essentially an archive of HTML files. So why not read ePub-files with a program which is specialized in displaying HTML-files and many people have already installed? That’s the idea which lead to the creation of EPUBReader as a Firefox addon. It runs on every platform Firefox does (Windows, MacOS X, Linux).
EPUBReader is available in 8 different languages: English, French, German, Greek, Lithuanian, Portuguese (European), Spanish, Traditional Chinese.
Coming soon: Hebrew, Italian, Russian, Dutch, Polish, Simplified Chinese and Tamil.
Your language not listed? It would be great, if you help with the translation, it just takes about 30 minutes. Please contact me here.
Above is a screenshot of an epub book I downloaded from Manybooks and displayed in the reader.
Definitely a good thing!