Requires b43-open installed https://www.parabola.nu/packages/libre/x86_64/linux-libre-firmware/

and the free b43 kernel module modprobed for a kernel that has support for it enabled (not linux-libre) https://wireless.wiki.kernel.org/en/users/Drivers/b43

dmesg

[   19.606197] b43-phy0: Broadcom 4312 WLAN found (core revision 15)
[   19.648160] b43-phy0: Found PHY: Analog 6, Type 5 (LP), Revision 1
[   19.648175] b43-phy0: Found Radio: Manuf 0x17F, ID 0x2062, Revision 2, Version 0
[   19.664601] Broadcom 43xx driver loaded [ Features: PNLS ]
[   19.666316] b43 ssb0:0: firmware: failed to load b43/ucode15.fw (-2)
[   19.666317] firmware_class: See https://wiki.debian.org/Firmware for information about missing firmware
[   19.666319] b43 ssb0:0: Direct firmware load for b43/ucode15.fw failed with error -2
[   19.666334] b43 ssb0:0: firmware: failed to load b43/ucode15.fw (-2)
[   19.666335] b43 ssb0:0: Direct firmware load for b43/ucode15.fw failed with error -2
[   19.666351] b43 ssb0:0: firmware: failed to load b43-open/ucode15.fw (-2)
[   19.666352] b43 ssb0:0: Direct firmware load for b43-open/ucode15.fw failed with error -2
[   19.666361] b43 ssb0:0: firmware: failed to load b43-open/ucode15.fw (-2)
[   19.666363] b43 ssb0:0: Direct firmware load for b43-open/ucode15.fw failed with error -2
[   19.666366] b43-phy0 ERROR: You must go to http://wireless.kernel.org/en/users/Drivers/b43#devicefirmware and download the correct firmware for this driver version. Please carefully read all instructions on this website.

lspci -vmmnn

Slot:	12:00.0
Class:	Network controller [0280]
Vendor:	Broadcom Corporation [14e4]
Device:	BCM4312 802.11b/g LP-PHY [4315]
SVendor:	Dell [1028]
SDevice:	Wireless 1397 WLAN Mini-Card [000c]
Rev:	01

lspci -nnk

01:00.0 Network controller [0280]: Broadcom Corporation BCM4312 802.11b/g LP-PHY [14e4:4315] (rev 01)
	Kernel driver in use: b43-pci-bridge
	Kernel modules: ssb

lspci -vvnnk

0c:00.0 Network controller [0280]: Broadcom Limited BCM4312 802.11b/g LP-PHY [14e4:4315] (rev 01)
	Subsystem: Dell Wireless 1397 WLAN Mini-Card [1028:000c]
	Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
	Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
	Latency: 0, Cache Line Size: 64 bytes
	Interrupt: pin A routed to IRQ 17
	Region 0: Memory at f69fc000 (64-bit, non-prefetchable) [size=16K]
	Capabilities: [40] Power Management version 3
		Flags: PMEClk- DSI- D1+ D2+ AuxCurrent=0mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
		Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=2 PME-
	Capabilities: [58] Vendor Specific Information: Len=78 <?>
	Capabilities: [e8] MSI: Enable- Count=1/1 Maskable- 64bit+
		Address: 0000000000000000  Data: 0000
	Capabilities: [d0] Express (v1) Endpoint, MSI 00
		DevCap:	MaxPayload 128 bytes, PhantFunc 0, Latency L0s <4us, L1 unlimited
			ExtTag+ AttnBtn- AttnInd- PwrInd- RBE+ FLReset- SlotPowerLimit 6.500W
		DevCtl:	Report errors: Correctable- Non-Fatal- Fatal- Unsupported-
			RlxdOrd- ExtTag+ PhantFunc- AuxPwr- NoSnoop-
			MaxPayload 128 bytes, MaxReadReq 128 bytes
		DevSta:	CorrErr+ UncorrErr- FatalErr- UnsuppReq+ AuxPwr- TransPend-
		LnkCap:	Port #0, Speed 2.5GT/s, Width x1, ASPM L0s L1, Exit Latency L0s <4us, L1 <64us
			ClockPM+ Surprise- LLActRep- BwNot- ASPMOptComp-
		LnkCtl:	ASPM L0s L1 Enabled; RCB 64 bytes Disabled- CommClk+
			ExtSynch- ClockPM+ AutWidDis- BWInt- AutBWInt-
		LnkSta:	Speed 2.5GT/s, Width x1, TrErr- Train- SlotClk+ DLActive- BWMgmt- ABWMgmt-
	Capabilities: [100 v1] Advanced Error Reporting
		UESta:	DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
		UEMsk:	DLP- SDES- TLP- FCP- CmpltTO- CmpltAbrt- UnxCmplt- RxOF- MalfTLP- ECRC- UnsupReq- ACSViol-
		UESvrt:	DLP+ SDES- TLP- FCP+ CmpltTO- CmpltAbrt- UnxCmplt- RxOF+ MalfTLP+ ECRC- UnsupReq- ACSViol-
		CESta:	RxErr+ BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr-
		CEMsk:	RxErr- BadTLP- BadDLLP- Rollover- Timeout- NonFatalErr+
		AERCap:	First Error Pointer: 14, GenCap+ CGenEn- ChkCap+ ChkEn-
	Capabilities: [13c v1] Virtual Channel
		Caps:	LPEVC=0 RefClk=100ns PATEntryBits=1
		Arb:	Fixed- WRR32- WRR64- WRR128-
		Ctrl:	ArbSelect=Fixed
		Status:	InProgress-
		VC0:	Caps:	PATOffset=00 MaxTimeSlots=1 RejSnoopTrans-
			Arb:	Fixed- WRR32- WRR64- WRR128- TWRR128- WRR256-
			Ctrl:	Enable+ ID=0 ArbSelect=Fixed TC/VC=01
			Status:	NegoPending- InProgress-
	Capabilities: [160 v1] Device Serial Number cf-9d-04-ff-ff-8b-70-1a
	Capabilities: [16c v1] Power Budgeting <?>
	Kernel driver in use: b43-pci-bridge
	Kernel modules: ssb