What is the difference between 詰まらない and 詰らない? Are they both valid words? Are they pronounced the same and do they have the same meaning?
As the comment indicates, it is possible that the first is tumaranai while the second is naziranai, the negative of the verb nazir-. While this is technically possible, it is not so simple. First, the kanzi for nazir- is not so common and few people will be able to read it. Second, tumaranai may be written as either 詰まらない or 詰らない. (For the rest of this response, I will be ignoring the nazir- reading.)
To expand on the second point, this is a matter of okurigana (送り仮名). There are a number of similar words in which there are multiple spellings: okonau (行なう, 行う), kotowaru (断わる, 断る), arawareru (現われる, 現れる) etc. This is documented by the Agency of Cultural Affairs here. This is a stylistic difference.