Try to disable hardware acceleration in Firefox (you need to close and restart Firefox).

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes
• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

Thanks, I've disabled it and restarted firefox, but there is no change for this issue... Find a screenshot attached

This can be a problem with the font that is used to display the text.

You can do a font test to see if you can identify corrupted font(s).

• http://browserspy.dk/fonts-flash.php?detail=1

You can try different default fonts and temporarily disable website fonts to test the selected default font.

• Tools > Options > Content : Fonts & Colors > Advanced
• [ ] "Allow pages to choose their own fonts, instead of my selections above"

Deselected also that option, still unsuccessfully, please find my second screenshot. Just out of curiosity, can you confirm that you correctly see the pdf at http://arxiv.org/pdf/physics/0612253.pdf (with spaces between words) from your firefox internal viewer? Thanks a lot for your help

I see the same with that PDF file, so this is likely a problem with the built-in PDF Viewer.
I can't copy the text to the clipboard either.

So the only option for this file would be to disable the built-in PDF Viewer and use another PDF Viewer (e.g. Adobe Reader).

The underlying Pdf.js issue has been fixed now. Look at https://github.com/mozilla/pdf.js/issues/4304 When will an updated version of Firefox - including the above fix - be available?

That is not easy to tell. There are a few Firefox development versions (Nightly, Aurora, Beta) that each have their own version of the PDF Viewer.
The current Nightly (30.0a1) build works for me, so that version appears to have the bug fix.
The latest Dev version works as well.
Older Firefox versions still show this issue.

The latest stable version of the PDF Viewer (pdfjs) as an extension:

• PDF Viewer: https://addons.mozilla.org/firefox/addon/pdfjs/

Latest development version of the PDF Viewer (pdfjs) extension:

• http://mozilla.github.io/pdf.js/extensions/firefox/pdf.js.xpi

Great ! I installed the xpi extension under development and it already includes the fix :-) (I hope I can get back to the stable version in case of need...)