I got the problem that the fonts in the firefox (ver. 25.0) looks really ugly and are a pain to read. When I save the PDF and open it with adobe reader, it's ok.
Here's a screenshot which shows a PDF in 100% scale.
You can try toggling Use hardware acceleration when available in Firefox Tools (Alt + T) > Options > Advanced > General, restart Firefox and check.
Another option you could try is to toggle Allow pages to choose their own fonts, ... in Firefox Tools (Alt + T) > Options > Content > Advanced and reload the PDF file. The corresponding about:config option is browser.display.use_document_fonts. Please note that this also affects fonts display in web pages.
See also the fourth comment. It looks like the improvement efforts are still ongoing and would be incrementally merged in future versions.