So it has been a little over week since I started using Firefox 4 Beta 13pre (64-Bit). This evening I was trying to read an article in Wikipedia but there appeared to be an issue with the character encoding. I switched the encoding from Unicode (UTF-8) to Western (ISO-8859-1) which fixed the issue for a moment but then reverted back to the Unicode encoding (although Western was still selected).
I was going to view the page using IE Tab Plus and got the message “IE Tab Plus can only work on Microsoft Windows.”. Now, looking at the add-ons manager in Firefox I do noticed that IE Tab Plus is not compatible with Firefox 4. I do happened to still have a 32-bit version installed (RC1) so I fired up this build on a clean profile. Loaded the page up in Wikipedia and same thing. However, when I went to install IE Tab Plus, I discovered another version that had been updated this month that does work with Firefox 4. After installing on the 64-bit version, I tried to switch into IE Tab mode and got the message “IE Tab Plus can only work on 32bit Microsoft Windows.” Least that error message makes more sense.
I searched the Firefox Builds forums but didn’t really come across anything that matched my issue. I went ahead and dropped a post on this issue, so we shall see what happens. I do know this is related to Firefox 4 as the page renders fine in Firefox 3.6
I guess it won’t help you, but the said article appears totally correct with FF4 (RC), for OS X. In 64 bit mode.
That narrows it down to being an issue with Windows (at the very least Win7).