Comment 42 for bug 1306849

Same issue with printing files from Firefox (and sometimes other applications) in Bionic with a Brother HL5250DN.
The used driver does not matter (BR-Script3, PCL, PS).
Related: https://bugzilla.redhat.com/show_bug.cgi?id=855009#c6

Solution:
Use the BR-Script3 driver, then edit (as root) the PPD for your printer (for me it's /etc/cups/ppd/Brother_HL-5250DN_Brotherscript3.ppd ).
The PPD starts with some comments, then the following content:

[...]
*%================================================
*% Copyright(C) 2005 Brother Industries, Ltd.
*% "Brother HL-5250DN BR-Script3"
*%================================================
*%==== General Information Keywords ========================
*FormatVersion: "4.3"
*FileVersion: "1.03"
*LanguageEncoding: ISOLatin1
*LanguageVersion: English
*Manufacturer: "Brother"
*PCFileName: "BR5250_2.PPD"
*Product: "(Brother HL-5250DN series)"
*PSVersion: "(3010.106) 5"
*ShortNickName: "Brother HL-5250DN BR-Script3"
*ModelName: "Brother HL-5250DN BR-Script3"
*NickName: "Brother HL-5250DN BR-Script3"
*1284DeviceID: "MFG:Brother;MDL:HL-5250DN series;CMD:PJL,PCL,PCLXL,POSTSCRIPT;"
*%==== Basic Device Capabilities =============
*LanguageLevel: "3"
[...]

Edit the line with "LanguageLevel" and change the "3" to "2". It should now look like this:
*LanguageLevel: "2"

Restart cups with "systemctl restart cups". Enjoy.