Thing is as a few eluded too, Texas mainly wanted independence from Mexico because Mexico banned slavery. Texas could not have survived as an independent nation without the US. Texas got independence to own slaves. Then joined the Union. Then the US has a civil war, slavery is abolished.
WI Mexico abolishes slavery after the US? WI they had the Irish Emigrants like a few mentioned. Texas would then not be able to survive between the two countries.
I always liked the argument between Bashir and O'Brien on DS9 about the Alamo.
Mexico abolishing slavery after the US is very hard to do. Unless you are talking about longterm changes in the Spanish Empire. The fact is that mulattos, slaves, and free blacks quite often served in the Mexican army even prior to indepence. In general race relations were quite different.
The problem with texas breaking away is that many parts of Mexico were breaking away at the same time. Mexico City simply couldn't govern it all. Canada on the other hand was organized like a federation and slowly given independence over time. This allowed different nationalities to be governed very fairly.
If somehow the Spanish with great fore sight decide to create an excellent developed federation, than Texas would be just like Quebec. Personally I don't see this happening due to medieval and materialistic nature of the Spanish Empire.