Why is firefox truncating a get request?

Question

PHP

Hello. Site on 1C-Bitrix, site encoding windows-1251. There is a filter on the page, it filters products by color (the value is in the #f9ced2 format). In chrome, everything works (the activ class is applied to the selected filter value using js (adds a border to the element). In safari, filtering occurs, but the activ class is not applied. Url in chrome and safari looks like ( ?arFilter_226_2549194743=Y&set_filter=%CF%EE %EA%E0%E7%E0%F2%FC ) In Firefox the filter does not work, if you look at the console it says "The form was submitted in windows-1251 encoding, in which it is impossible to encode all Unicode characters, so the data entered by the user is could be malformed.To avoid this problem, the page should be modified so that the form is submitted in UTF-8 encoding,
If you add accept-charset=utf-8 to the form, nothing changes (concluded that the problem is not related to the encoding at all).
Question:
1) Why is there no arFilter in the url in firefox ...... (in fact, the value by which the filtering will take place is written to this array)
2) Why is class not used in safari?