Unable to select complete sentences

I can’t highlight complete sentences, unless they are very short. When I try to select a long phrase there are always some words missing.

I’m using the latest Firefox in a Linux desktop.

Hi solidgitarius,
You can select maximum 9 words at once. So if you are trying to highlight a phrase which have more than 9 words, that’s why you are unable to do so.

I am able to save more than 9 words on my phone but my desktop does not allow it.