Search for keyHash and save the value somewhere, in case the .log file gets wiped (in fact, save a copy of the entire .log file somewhere safe). Check the kdfIterations value as well, which presumably will equal 100000.
Next, go to this page, and use your browser to save the HTML file (source code) of that page. Then use any text editor (e.g., Notepad) to edit Line 481 of the HTML file, changing the third argument of the pbkdf2 function from 1 to 2, so that it looks like this:
self.masterKeyHash = await pbkdf2(newValue.arr.buffer, self.masterPasswordBuffer, 2, 256)
(note that the second to last number at the end of the expression should read 2 instead of 1). Save the HTML file, and then open the edited HTML file in any browser that has Javascript enabled. The form should look just like the original web form, but the original web form will not give you the correct values for the purposes required.
On the modified form, enter your email, the kdfIterations value from the .log file, and your Master Password (the way you remember it). Compare the Master Password Hash that was calculated on the webpage to the value of keyHash that you copied from the .log file. If they match, then you have entered the correct Master Password. If they don’t match, you can keep guessing until you get a match.
If you are unable to guess your Master Password using the above method, but if it there is only part of the password that you are unsure of, then you can automate the brute-force guessing process using a tool like Hashcat.