I found it: 2534 Woodland Ave. It's still there, and looks about the same as it did in 1910! At least according to Zillow. How did I do it? For a few minutes, I thought it was one of those things with just not quite enough info to go on. 

Mrs. D.B. Warren (nee Huntington) lived for a time in Ontario, but was buried in Cook County, Illinois (according to Find-a-Grave.). Frustratingly, her memorial page is only linked to her parents and siblings, no spouse or kids. She was married to David, but how does one find the name of who her daughter married, hence giving us a new last name for Lillian (the sender?). Hmmm. I tried a few things, no go. Then, I decided to check if anyone else in that cemetery was related to her, just not linked. Hmm, there was a Lillian Warren buried there who died in 1923. Well, possible based on the 1910 postcard. But why was her last name Warren if she was married? 

But, isn't there a theory out there that not everything needs to be hard? So I stuck Lillian Warren into Ancestry, and there she popped up, living with her husband George Warren in 1910, at 2534 Woodland Ave. She was 26, her husband 32, and they had a 27 year old boarder named John Plotnicky. Huh, that matches what Paul thought the address might be....I checked, and bingo! 

So! Either Lillian happened to marry someone with the same last name, or she is addressing her mother-in-law as "Mother dear." Satisfying to find the actual house!