Hi. I'm running Mac Tiger 10.4.11 so can't upgrade Firefox.

Question

How long will I be able to use the old version? Is security still OK?

Answer 1

Support for Firefox 3.6.x is expected to end in the first quarter of 2012.
Beginning with Firefox 4, Macs must have an Intel x86 processor (which you do have) '''''<u>and</u>''''' OSX 10.5 or higher ('''''yours is OSX 10.4''''').
*System Requirements: [https://www.mozilla.org/en-US/firefox/4.0/system-requirements/index.html Firefox 4] [https://www.mozilla.org/en-US/firefox/5.0/system-requirements/index.html Firefox 5] [https://www.mozilla.org/en-US/firefox/6.0/system-requirements/index.html Firefox 6] [https://www.mozilla.org/en-US/firefox/7.0/system-requirements/index.html Firefox 7] [https://www.mozilla.org/en-US/firefox/8.0/system-requirements/index.html Firefox 8]
'''''The good news''''' is that Firefox 3.6.24 still supports OSX 10.4 and Macs with the PPC Mac architecture. '''''The bad news''''' is that support for the Firefox 3.6.x series is likely to end in the first quarter of 2012.
*System requirements: [https://www.mozilla.org/en-US/firefox/3.6/system-requirements/index.html Firefox 3.6.x]
*Download Firefox 3.6.24: https://www.mozilla.org/en-US/firefox/all-older.html
*Also see --> http://support.mozilla.com/en-US/kb/Installing+Firefox+on+Mac
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Not related to your question, but...
You may need to update some plug-ins. Check your plug-ins and update as necessary:
*Plug-in check --> http://www.mozilla.org/en-US/plugincheck/
*Adobe Shockwave for Director Netscape plug-in: [https://support.mozilla.com/en-US/kb/Using%20the%20Shockwave%20plugin%20with%20Firefox#w_installing-shockwave Installing ('''''or Updating''''') the Shockwave plugin with Firefox]
*Adobe PDF Plug-In For Firefox and Netscape: [https://support.mozilla.com/en-US/kb/Using%20the%20Adobe%20Reader%20plugin%20with%20Firefox#w_installing-and-updating-adobe-reader Installing/Updating Adobe Reader in Firefox]
*'''''Shockwave Flash''''' (Adobe Flash or Flash): [https://support.mozilla.com/en-US/kb/Managing%20the%20Flash%20plugin#w_updating-flash Updating Flash in Firefox]
*Next Generation Java Plug-in for Mozilla browsers: [https://support.mozilla.com/en-US/kb/Using%20the%20Java%20plugin%20with%20Firefox#w_installing-or-updating-java Installing or Updating Java in Firefox]