What's the Name of That Book??? discussion

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Scruffy
SOLVED: Children's/YA
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SOLVED. Children's/ YA Fiction. A stray dog and her life on the streets (possibly also made into an obscure animated movie?)? May have been written in mid 20th century or 1970s? [s]
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It starts out focused on the mother dog, who used to belong to humans and remains in their old house after they left her behind. Later the story focuses on her puppy. The book is NOT "A Dog's Life" by Ann M. Martin. It was published much earlier than that, I think (unsure, but perhaps written sometime in the 1960s? I may be wrong).
Spoilers will follow, as I'll try and post all the details I remember.
(view spoiler)[
The book begins with a mother dog (who for some reason I imagined as a border collie, but that might not be correct) who is abandoned by her humans and stays in the house they used to live in (I say abandoned, but I recall that a neighbor or friend might have initially been feeding her). She was taught not to go upstairs by her people, so she doesn’t go upstairs even when the people are gone. Then she has a puppy (who I feel like was some spaniel-esque brown dog?). The house catches on fire and the mother dog finally goes upstairs to save her puppy from the flames, and she is rescued by firefighters. I remember the firefighter being one of the kind humans she meets (maybe feeding her pieces of his sandwich at one point).
Then somehow they escape or run away (I think the mother dog was still intent on finding her former people) and are back to wandering by themselves. I remember the mother dog got shot (probably by a farmer?), and then the pup is alone until she comes across a group of strays. The group includes a collie that adopted a boot after her pups died, and a mutt (grey?) dog that ends up being the main dog’s love interest, named something generic (maybe Butch)? The love interest dog is the leader of this group of strays.
I also remember the collie getting hit by a truck but surviving to be adopted by the man that hit her, and the other strays getting caught by dog catchers and taken to the pound. On the love interest’s dog’s last day, someone wants to adopt the main character dog, but she is so upset while they are taking the grey dog back to be killed that the people adopt him as well. Happily ever after, yay! I also remember them having puppies, so this would have been written before animal shelters began spaying/neutering the animals they adopt out.
I've tried searching plot points on Google (such as "Mother dog escapes fire" or "collie adopts a boot") but still found nothing. (hide spoiler)]
I'm really hoping someone knows the book I'm talking about, as this has been driving me crazy for some time! Thank you for the help!
(This is my first post here, so I apologize if I made any mistakes!)