A couple of announcements provide fresh reasons for planning to replace that old Mac you've been hanging on to.

Mozilla has decided that Mac OS X 10.5 will be a minimum requirement for Gecko 1.9.3. Gecko is the HTML engine used in Mozilla software such as Firefox, and version 1.9.3 will be used in a successor to the recently released Firefox 3.6.

Part of the reason for dropping support for Mac OS X 10.4 is that Apple has already stopped delivering security updates for it. The date on which no new security updates will be released for Firefox 3.6 has not yet been determined.

The main difference in the system requirements for Mac OS X 10.4 and 10.5 is that the former will run on G3 and slower G4 processors. Older G3 systems such as the 'Lombard' PowerBook G3 are officially limited to Mac OS X 10.3.

Talking of Mac OS X 10.3 brings us to Google's decision to drop support for Safari 2 in YouTube and Google Apps from March. Safari 2 is part of Mac OS X 10.3.

Support for Internet Explorer 6, Firefox 2 and Chrome 3 are also going from Google Apps, so users of old Macs shouldn't feel victimised.

While Safari 2 users will still be able to watch YouTube videos, "some features that we launch in the weeks and months ahead may not be supported", Google officials have warned. The Google Apps situation is more serious, as "key functionality" as well as new features "won't work properly".

You knew the usefulness of that old Mac has been decreasing, and Mozilla and Google have added some nails to the coffin. You may be able install a newer version of Mac OS X to extend its life, but eBay 'buy it now' prices for retail copies of Mac OS X 10.4 are over $100, and that might be better applied to the price of a new - or at least newer - computer.

If you can't afford $1299 for the cheapest MacBook, it's not that hard to find a used one for perhaps half as much.

These announcements shouldn't be a cause for panic - Mozilla's is particularly forward-looking - but they do provide additional justifications for that upgrade you wanted.