Tifa first mentions her committment to planet ecology when Cloud tried to leave AVALANCHE over not getting paid. She begs him to stay, mostly because of their childhood memories, but she also mentions that the planet is dying. Tifa participates in the bombing of the Sector 5 reactor, but then is remorseful, due to the noncombatants that were killed in the explosion. However, she still does fight the Shinra, she still is fine with the assassination of the of the Shinra leaders Heidegger, Scarlet, and Professor Hojo, along with some Shinra Soldier 1st class random battle enemies that can be encountered in that area. that takes place near the end of Disc 2. She just fights in a way that does not kill noncombatants. Tifa also respects the condors in Fort Condor,
(At Fort Condor) Tifa: Condor's Eggs... A real work of nature. We've got to protect the life of the planet.
She also loves the birds in Mt Corel (as do Barret and Aeris, depending on who is in your party at that time), but she also makes the distinction that Aeris is a human not an animal, in an argument with Professor Hojo, who thought of Aeris as an animal. These two points contradict each other, and the best (somewhat conjectural) answer I could think of to explain this is that maybe Tifa doesn't think birds are animals either.
Tifa also understands planet ecology fairly well, and learned from Bugenhagen at Cosmo Canyon, as shown here:
(After the Shinra Rocketship launched, and the main characters recovered the Huge Materia)
Red XIII: Huh!? Did you...... hear something?
Tifa: The planet's scream...... or Meteor...? Is it this planet?
Cloud: Hey, how do we know that this is really the planet's scream?
Tifa: Did you forget? Bugenhagen told us.
Tifa also mentions the planet in the Last Night Under Highwind scene (indicating she too still fights for the planet after Cloud's "Reasons for Fighting" speech),
(Cloud and Tifa under the Highwind at the end of Disc 2)
Tifa: Cloud? Do you think the planet heard us
too? Do you think that someone out there is watching over us and
thinking, "You are doing a great job..." ?
(Taken from the Cathy Okada translation: http://www.ff7citadel.com/gameinfo/textfiles/reasons.txt. The original translation had numerous innacuracies, so this version is valid in my opinion too.)
