http://mmafighting.com looks fine here. Try clearing your cache and cookies.

1. Open the Tools menu, then select Clear Recent History... .
2. In the Time Range to clear: drop-down, select Everything.
3. Click the arrow next to Details to display the list of items that can be cleared.
4. Select both Cookies and Cache.
5. Click Clear Now .

Can you attach a screenshot?

Use a compressed image type like PNG or JPG to save the screenshot and make sure that you do not exceed the maximum file size (200 KB) and do not click Preview after you have attached the image.

I opened an article on the world cup from aol and no text appeared but when I right clicked and selected all the text became highlighted.  not quite sure how to do a screen shot of this for you to see

when I go to the suggested site for screen shots at Wikipedia the text is largely missing only the link text items show until I right click select all then the underlying text appears.

Perhaps your color settings are different. See Website colors are wrong.

If that doesn't work, we'll need some more info about your Firefox setup. Go to Help > Troubleshooting Information, then click Copy all to Clipboard. Open a reply to this post, and go to Edit > Paste to paste the info from your Troubleshooting Information page.

 Application Basics

       Name
       Firefox

       Version
       3.6.3

       Profile Directory

         Open Containing Folder

       Installed Plugins

         about:plugins

       Build Configuration

         about:buildconfig

 Extensions

       Name

       Version

       Enabled

       ID

       AVG Safe Search
       9.0.0.825
       false
       {3f963a5b-e555-4543-90e2-c3908898db71}

       AVG Security Toolbar
       4.002.023.004
       false
       avg@igeared

 Modified Preferences

     Name

     Value

       browser.places.importBookmarksHTML
       false

       browser.places.smartBookmarksVersion
       2

       browser.startup.homepage_override.mstone
       rv:1.9.2.3

       extensions.lastAppVersion
       3.6.3

       font.size.variable.x-western
       18

       keyword.URL
       http://us.yhs.search.yahoo.com/avg/search?fr=yhs-avg&type=yahoo_avg_hs2-tb-web_us&p=

       network.cookie.prefsMigrated
       true

       places.last_vacuum
       1271818653

       print.print_printer
       HP Photosmart C6100 series

       print.printer_HP_Photosmart_C6100_series.print_bgcolor
       false

       print.printer_HP_Photosmart_C6100_series.print_bgimages
       false

       print.printer_HP_Photosmart_C6100_series.print_command

       print.printer_HP_Photosmart_C6100_series.print_downloadfonts
       false

       print.printer_HP_Photosmart_C6100_series.print_edge_bottom
       0

       print.printer_HP_Photosmart_C6100_series.print_edge_left
       0

       print.printer_HP_Photosmart_C6100_series.print_edge_right
       0

       print.printer_HP_Photosmart_C6100_series.print_edge_top
       0

       print.printer_HP_Photosmart_C6100_series.print_evenpages
       true

       print.printer_HP_Photosmart_C6100_series.print_footercenter

       print.printer_HP_Photosmart_C6100_series.print_footerleft
       &PT

       print.printer_HP_Photosmart_C6100_series.print_footerright
       &D

       print.printer_HP_Photosmart_C6100_series.print_headercenter

       print.printer_HP_Photosmart_C6100_series.print_headerleft
       &T

       print.printer_HP_Photosmart_C6100_series.print_headerright
       &U

       print.printer_HP_Photosmart_C6100_series.print_in_color
       true

       print.printer_HP_Photosmart_C6100_series.print_margin_bottom
       0.5

       print.printer_HP_Photosmart_C6100_series.print_margin_left
       0.5

       print.printer_HP_Photosmart_C6100_series.print_margin_right
       0.5

       print.printer_HP_Photosmart_C6100_series.print_margin_top
       0.5

       print.printer_HP_Photosmart_C6100_series.print_oddpages
       true

       print.printer_HP_Photosmart_C6100_series.print_orientation
       0

       print.printer_HP_Photosmart_C6100_series.print_pagedelay
       500

       print.printer_HP_Photosmart_C6100_series.print_paper_data
       1

       print.printer_HP_Photosmart_C6100_series.print_paper_height
       11.00

       print.printer_HP_Photosmart_C6100_series.print_paper_size_type
       0

       print.printer_HP_Photosmart_C6100_series.print_paper_size_unit
       0

       print.printer_HP_Photosmart_C6100_series.print_paper_width
       8.50

       print.printer_HP_Photosmart_C6100_series.print_reversed
       false

       print.printer_HP_Photosmart_C6100_series.print_scaling
       1.00

       print.printer_HP_Photosmart_C6100_series.print_shrink_to_fit
       true

       print.printer_HP_Photosmart_C6100_series.print_to_file
       false

       print.printer_HP_Photosmart_C6100_series.print_unwriteable_margin_bottom
       0

       print.printer_HP_Photosmart_C6100_series.print_unwriteable_margin_left
       0

       print.printer_HP_Photosmart_C6100_series.print_unwriteable_margin_right
       0

       print.printer_HP_Photosmart_C6100_series.print_unwriteable_margin_top
       0

       privacy.sanitize.migrateFx3Prefs
       true

       privacy.sanitize.timeSpan
       0

       security.warn_viewing_mixed
       false

If text becomes visible if you select it then the text color is the same as the background color or the text color is transparent. Did you check the settings in Tools > Options > Content : Fonts & Colors: Colors ?

See also Website colors are wrong and Websites look wrong or appear differently than they should --- Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window to disable all extensions. You have to close and restart Firefox after each change via "File > Exit" (on Mac: "Firefox > Quit")