Version 5.4 no longer supported

[stevef]

Why was Firefox browser replaced with Chrome browser in later versions 6.6 and 7.6 ?

Mozilla made Firefox a snap package which was seen as a negative.
If you are prepared to have snap on your system, the snap version of Firefox is available through Lite Software.
Alternatively instructions are given to use a non-snap version of Firefox.

The size of each install is increasing :
version 5.4 1.4GB
version 6.6 2.4GB
Version 7.6 2.9GB

Yes.

For me Chrome is a negative and adding more features is a negative.

Browser preferences are clearly subjective.  Before the release of 6.0 there was an active thread discussing what Linux Lite 6.0 default browser should be - ultimately the choice was Jerry's. Features added since 5.x include language packages and accessibilty options which are intended to allow more people to use Linux Lite.  Other features have been added because they have been requested but I guess you can't please everyone.

I'm happy with Version 5.4 and only need BASIC features which include Firefox, Libreoffice, pdf viewer and plays videos.

As you are aware 5.4 is unsupported but there is no obligation to do anything about it.

My Macbook Air computer is 10 years old.

Is Linux Lite still the best option for me OR can you recommend another LIGHTWEIGHT version of Linux ?

This is your decision only and discussion isn't appropriate on this forum.
See the forum rules -

Other Distros - there should not be any posts asking for help with other distros. This includes requests for basic support, requests for recommendations, comparisons, benchmarks and other common sense examples. In this Forum you will receive the help you need regarding Linux Lite but please do us the basic courtesy of posting in places on the internet where support for other distros is more specific like their user Forums or more broadly, places like Linux Questions dot org.

Good Luck